*% 













V 

■ . 









*> 



v- 






^ V* 









"> 






^ c 



















' 


-0' X 


w 




i5 u 










«K 



A^' '^A 









* <£ 



%. 









'" ft '/ 






\ v 







SIXINCHACHROMATIG TELESCOPE 

MADEBYMR.HENRYFITZ OFTHE CITY OF NEW YORK, 
AND MOUNTED EQUATORIALLY BY MESSRS. GREGG fy RUPP 
OF THE SAME CITY. IT IS 8 FEET IN LENGTH,^ T HE 
TOTAL COST OF THE INSTRUMENT ABQUT$IOOO. IT SHOW< 

HE MOON ^PLANETS WITH GREAT SHARPNESS.THE 
5TH. £6TH. STARS IN THE TRAPEZIUM v OF 9, ORIONIS.& 
SEPARATES E, ARIETIS, 36ANDR0MEDAE, AND OTHER CLOSf 
STARS OF THE SAME CLASS. IT IS NOW EJECTED IN THE 
OBSERVATORY OF LEWIS M. RUTHERFORD, ESQ. INTHE 

CITY OF NEW YORK. 



H&cfli. cJ^jAix-ty^ i-L.(?n 



PRIMARY ASTRONOMY, 



FOR 



SCHOOLS AND FAMILIES: 



ADAPTED TO THE CAPACITY OF YOUTH, AND ILLUSTRATED 
BY NEARLY 



TWO HUNDRED ENGRAVINGS. 



BY HIRAM MATTISON, 



PROFESSOR OF NATURAL PHILOSOPHY AND ASTRONOMY IN THE FALLEY SEMINARY I 
AUTHOR OF THE ELEMENTARY ASTRONOMY, ASTRONOMICAL MAPS, 
ETC. ETC. ^ 



NEW YORK: 
HUNTINGTON AND SAVAGE, MASON AND LAW, 

23 PARK ROW, (Opposite the Astor House.) 
1851. 



zL * 



& 






Entered according to Act of Congress, in the year 185 1, 

By HIRAM MATTISON, 

In the Clerk's Office of the District Court for the Southern District of New York. 



0* 

y L 



V 



k 



Stereotyi)ed by 
RICHARD C. VALENTINE, 

45 Gold-st., New York. 



CONTENTS. 



PART FIRST. 

PRELIMINARY OBSERVATIONS AND DEFINITIONS. 

PAGE 

I. History of Astronomy 5 

II. The Modern, or Copernican System 9 

IIL Geometrical Definitions 14 

IV. Of Lines and Angles 18 

V. Of the Circle and the Ellipse 20 

VL The Terrestrial and Celestial Spheres 24 



VII. 

VIII. 

IX. 

X. 

XI. 

XII. 

XIII. 

XIV. 

XV. 

XVL 

XVII. 

XVIII. 

XIX. 

XX. 

XXI. 

XXII. 

XXIIL 

XXIV. 

XXV. 

XXVL 

XXVII. 

XXVIII. 

XXIX. 

XXX. 

XXXI. 



PART SECOND. 

OF THE SOLAR SYSTEM. 

Bodies that compose the System 31 

Names of the Planets, Signs, <fec. 33 

Distances of the Planets from the Sun 38 

Light and Heat of the several Planets 40 

Magnitude, Density, and Gravitation of the Planets 42 

Revolution of the Planets around the Sun 46 

Aspects, Sidereal and Synodic Revolutions, &c 50 

Direct and Retrograde Motions, Planets Stationary, &c... 53 

Diurnal Revolutions of the Planets, Time, &c 51 

The Ecliptic, Zodiac, Signs, Longitude, &c 63 

Form and Position of the Planetary Orbits, Nodes, &c. ... 69 

Of Transits 73 

The Sun's apparent Motions, the Seasons, <fec 76 

Seasons of the different Planets, Telescopic Views, &c 82 

Telescopic Views of the Planets continued 87 

Of the Secondary Planets. 93 

Revolution of the Moon around the Earth 96 

The Moon's Changes 101 

Day and Night, Seasons, <fec, of the Moon 106 

Eclipses of the Sun 113 

Eclipses of the Moon 120 

Satellites of Jupiter, Eclipses, <fec 123 

Satellites of Saturn, Herschel, and Neptune 128 

Of the Tides 131 

Of Spring and Neap Tides 135 



SUGGESTIONS TO TEACHERS, ETC. 



XXXII Of Comets 138 

XXXIII. Of the Sun 145 

XXXIV. General Remarks upon the Solar System.... 152 



PART THIRD. 

OF THE SIDEREAL HEAVENS. 



XXXV. The Fixed Stars — their Number, Distances, &o 155 

XXXVI. Double, Variable, and Temporary Stars 159 

XXXVII. Clusters of Stars and Nebulae 161 

XXXVIII. Of the Atmosphere, Winds, Clouds, Storms, &c 165 



SUGGESTIONS TO TEACHERS AID STUDENTS. 

1. The coarse print should generally be committed to memory, as 
an answer to the questions. 

2. The fine print and the cuts should be carefully studied ; and 
the student should be prepared, if called upon, to make drawings 
similar to those found in the lesson upon the blackboard. 

3. In studying the illustrations, the pupil should generally 
imagine himself to be looking South — the top of the book being 
North ; the bottom South ; the left hand East ; and the right hand 
West. 

4. The teacher should call upon the several members of the 
class, in succession, to sketch the diagrams as they occur in the 
lesson, and to show how they illustrate the point under considera- 
tion — this explanation being in all cases extemporaneous. 

5. The Pronunciation, Definition, and Derivation of terms should 
receive special attention ; and to facilitate such inquiries, important 
words are not only pronounced and defined, but are traced to their 
original source. 

6. Finally, whenever practicable, let the objects or phenomena 
described in the lessons be looked up and observed as they really 
appear in nature. This will invest the subject with new interest, 
and will excite in the mind of the student a desire to know more of 
this most wonderful and most sublime of all human studies. 

New York, January, 1851. 



PRIMARY ASTRONOMY, 



PART I. 

PRELIMINARY OBSERVATIONS AND DEFINITIONS. 



LESSON I. 

HISTORY OF ASTRONOMY. 

1. Question. — What is Astronomy ? 

Answer. — It is the science of the Heavenly Bodies — 
the Sun, Moon, Planets, Comets, and Stars. 

2. What does it teach respecting these bodies ? 

Their names, distances, magnitudes, and motions, and 
the causes of Day and Night, the Seasons, Eclipses, 
Tides, and various other phenomena.* 

3. Was Astronomy known to the ancients? 

It was; and was taught in Egypt, Chaldea, India, 
China, and Greece long before the birth of Christ. 

4. What was the occupation of the first Astronomers ? 
They were shepherds and herdsmen. 

5. By what were they led to this study? 

By observing the movements of the sun, moon, and 
stars, while watching their flocks from year to year in 
the open fields, f 



* Phe-nom'-e-na, appearances ; things presented to the eye, or events 
seen to take place. The singular of this term is Phe-nom-e-non. 
f Norton's Elementary Treatise, page 2. 

1* 



PRIMARY ASTRONOMY. 



ANCIENT ASTRONOMERS OBSERVING THE HEAVENS. 




6. Where do modern Astronomers make their observations 1 
In what are called Observatories. 

[An observatory is a place erected for the purpose of observing the heavens. The 
most celebrated in this country is the one erected by Professor Mitchel on Mount 
Adams, Cincinnati. Besides this, there is one at Washington, one at Philadelphia, 
one at Cambridge, Mass., &c. An effort is now being made (1850) to erect one on 
Brooklyn Hights, Long Island. A good private observatory, owned by Lewis M. 
Rutherford, Esq., may be seen in the rear of his residence, corner of Second Avenue 
and Eleventh street, New York City.] 

BR. DICK'S OBSERVATORY, NEAR DUNDEE, SCOTLAND. 




7, What other advantage have modern Astronomers ? 
They are assisted by many useful instruments, the 
most important of which is the Telescope* 

* From the Greek tele, at a distance, and skopeo, to see. 



PRIMARY ASTROXOMY. 



MODERN ASTRONOMERS USING 
A TELESCOPE. 




[This is a picture of the Great Refracting 
Telescope of the Cincinnati Observatory. It is 
reduced and copied from a cut in the first num- 
ber of the Sidereal Messenger* July, 1846. The 
original (which is an enlargement of a daguerre- 
otype) is exceedingly accurate and beautiful, and 
our artist has given us an exact miniature of it in 
the page before you. 

This Telescope has an object-glass (placed in 
the top of the tube toward the objects to be seen) 
which is twelve inches in diameter. The tube, 
which is seventeen feet long, and the Telescope, 
alone cost $9,437. It was made at Munich, in 
Germany.] 

§. What did the ancients think of the 

form of the earth ? 

Instead of a globe inhabited 

on all sides, they believed it to 

be a plane, inhabited on one 

side only. 

9. What did they think of her motions, and those of the heavenly 
bodies ? 

They believed the earth to be at rest in the center of 
the universe, and that the sun, moon, and stars actually 
revolved around her, from east to west, as they appear 
to do, every twenty-four hours. 

[It is by no means strange that the ancients were thus deceived by appearances. 
Seeing but a small portion of the earth at any one time, they were easily misled in re- 
gard to its form ; and having no idea of its revolution, it was natural to suppose that 
the apparent daily revolution of the heavenly bodies westward was real. In the same 
manner persons often attribute their own motions to bodies that are at rest, especially 
when carried swiftly forward without any apparent cause (as when one travels in a 
steamboat or railway car), and when for a time they forget their own motions.] 

10. What other notions had the ancients in respect to the figure of 
the earth ? 

That it was longest from east to west. 

[They observed that traveling east or west had no effect upon the apparent position 

of the stars, while going north and south did. Hence the conclusion that the earth 

was narrowest north and south. This erroneous idea was the origin of our modern 

terms, latitude and longitude, the last of which means length, and the former 

I breadth.'] 

11. What was the ancient system of Astronomy called, and why? 



8 



PRIMARY ASTRONOMY. 



The Ptolemaic* Theory, from Ptolemy, f an Egyptian 
philosopher, who nourished in the second century of the 
Christian era. 

[1. The word Ptolemy was a common name for the successive Egyptian princes, 
as Pharaoh:): was for the kings of Egypt in the days of Moses. 

PTOLEMAIC THEORY OF THE STRUCTURE OF THE UNIVERSE. 




2. The ancients supposed the earth to be surrounded by eight crystalline arches 
or spheres, one within the other, in which the sun, moon, and stars were set. In this 
way they accounted for their not falling down upon the earth, while the light of the 
most distant of the stars could pass through the transparent crystal till it reached 
the earth. The moon was supposed to be in the first sphere, the sun in the fourth, 
and the fixed stars in the eighth or highest sphere. The cut is intended to illustrate 
all these particulars. 

3. This erroneous theory was embarrassed by numerous and pressing difficulties, one 
of which was to ascertain what upheld the earth. Tn the effort to explain this mys- 
tery several remarkable notions were adopted, one of which is represented in the cut. 
The earth is seen as a plane, resting upon the head of a huge serpent, which, in turn, 
is upheld by a tortoise. The sun, moon, and stars are shown revolving around the 
earth. This absurd theory was generally believed to be correct till about the middle 
of the sixteenth century, or 300 years ago.] 



* Pronounced Tol-e-ma' 



f Tole'-mv. 



X Fa'-ro. 



PRIMARY ASTRONOMY. 



LESSOR II. 

THE MODERN, OK COPEENICAN" SYSTEM. 




12. Describe the modern or true theory. 
It represents the sun as trie fixed center, around 
which the earth and other planets revolve. 

IS. What does it teach respecting the form of the earth? 

That it is a Sphere or Globe. 

14. To what does it attribute the apparent westward revolution of 
the sun, moon, and stars 1 

To the actual revolution of the earth eastward upon 
its axis every twenty-four hours. 



10 PRIMARY ASTRONOMY. 

[If a fly were placed upon an artificial globe in a room where several lamps were 
suspended, and the globe were made to revolve, he would be likely to conclude, if he 
could reason, that the lamps revolved around him, especially if he was firmly fixed 
upon the globe, and had no suspicion of his own motion. In the same manner the 
ancients concluded the heavens revolved instead of themselves.] 

15. What is the modern system of Astronomy called, and why 1 
The Copernican Theory, after Copernicus, a Prus- 
sian* astronomer, who tanght it. 

16. When and where was Copernicus born ? 
In Thorn, in Prussia, in 1473. 

17. When did he begin to propagate his theory? 

About the year 1510. 

[This was about 18 years after the discovery of America by Columbus, or 340 years 
ago.] 

1§. Was this theory entirely unknown before 1 

It is supposed to have been taught by Pythagoras^ 

a Greek philosopher, about 500 years before Christ. 

19. Who next taught it after Copernicus? 
Galileo, the inventor of the Telescope. 

20. How were his doctrines received? 

They were pronounced erroneous, and he was obliged 
to renounce them. 

[The following is his renunciation, made June 28, 1633 : a I, Galileo, in the seventieth 
year of my age, on bended knees before your eminences, having before my eyes and 
touching with my hands the Holy Gospels, I curse and detest the error of the earth's 
movement." As he left the court after this forced renunciation, he is said to have 
stamped upon the earth, and exclaimed, " It does move after all !" Ten years after 
this we find him in prison at Rome for the same supposed error.] 

21. What proof have we that the earth is round, from the appear- 
ance of the heavenly bodies ? 

The obvious figure of the sun, moon, and stars seems 
to indicate that all worlds are spherical, and conse- 
quently that the earth is a globe. 

[This reason for believing the earth to be a sphere would have little weight where 
the true system of Astronomy was not first understood ; for it assumes that the sun, 

* Pru'-shan. f Py-thag'-o-eas. 



PKIMARY ASTRONOMY. 11 

moon, and stars, which are seen to be round, are worlds; and as all worlds of 
which we can see the shape are round, the inference is that the earth also is 
round.] 

22. What proof from the appearance of the earth? s surface ? 
The convexity* of the earth's surface shows it to be 

a globe or ball. 

23. How do we know that the earth's surface is convex ? 

It is evident from the fact that the masts of a ship, 
approaching us from sea, are always seen before the 
hull ; and the higher the observer is on the shore, the 
more of the ship will he see.f 

CONVEXITY OF THE EARTH'S SURFACE. 



[Here the observer upon the shore at A sees only the top-masts of the ehip, while 
the man standing upon the pillar at B sees the masts and sails, and part of the hull. 
Now if the water between A and the ship were exactly flat instead of convex, the 
vision of A would extend along the line C, and he could see the whole ship as well as 
B. The advantage of B over A, in consequence of his elevation, shows that the sur- 
face of the waters is convex between A and the ship.] 

24. What additional proof have we that the earth's surface is con- 
vex* 

In constructing aqueducts:]: it is found necessary to 
have the middle elevated a little above the plane of the 
ends, otherwise the water will run over in the middle 
before the ends are full. 



* Con-vex'-i-ty, swelling toward a globular form ; roundness. 

■{■ It would be well, where there is time, during recitation, for some 
member of the Class to draw this and similar figures used for illustration 
upon the Blackboard, and explain the same in the hearing of the 
Teacher. 

\ Ak'-we-duct, a structure for conveying water from one place to 
another, as across rivers, &c 



12 



PRIMARY ASTRONOMY. 



CONVEXITY DISCOVERED IN THE BUILDING OF AQUEDUCTS. 




MM©- 



ROTUNDITY OF THE EAKTH. 



[In the cut, the top of the aqueduct is perfectly flat ; but the water, on being let in, 
conforms to the general figure of the globe, swells upward in the middle, and runs 
over. This proves the surface of the water to be convex. The amount of this con- 
vexity is found to be about eight inches to a mile.] 

25. What fourth proof can you offer that the earth is a globe or 
balll 

As we go north upon its surface the stars in the south 
seem to go down, till they sink below the horizon.* 

[The same is true of the southern stars when we 
go north, and this is precisely what would occur if 
the earth were a globe. In the figure, the observer 
at A sees the star C elevated about 40 degrees 
above his horizon. As he recedes toward his 
position at B, the star seems to sink toward the 
earth, till at length it goes down or out of sight. 
The convex surface of the earth is now be- 
tween the star and the observer. This eleva- 
tion and depression of the North Star, caused 
by the observer's change of position, enables the 
mariner to determine bis latitude upon the track- 
less deep ; for as every degree he sails north- 
ward causes the Pole Star to rise one degree, 
it follows that the apparent altitude of the star and the latitude of the observer 
are the same; the former being ascertained, the latter is at once known.] 

26. What is the fifth proof that the earth is a globe? 

The shadow of the earth, when seen npon the surface 
of the moon, in an eclipse, is convex ; which shows the 
earth, from which the shadow is projected, to be convex 

also. 

[1. It is not pretended that this reasoning is altogether logical and conclusive, for 
any circular object, like a common plate, will cast a convex shadow, if held in a cer- 
tain position. Still, as the earth revolves, and in all positions casts a convex shadow, 
it is strong presumptive proof that the earth is a sphere, as no other form would 
always cast such a shadow under the same circumstances. 




* Ho-Rl'-ZON. 



PEIMAEY ASTEONOMY. 



13 




2. In this cut the shad- convexity of the earth's shadow. 

ow of different objects is 
exhibited. At A the ob- 
ject is of a cubical form, 
and casts a shadow ac- 
cordingly upon the moon 
at the right. At B the ob- 
ject is triangular or pris- 
matic, and the shadow is 
triangular; but the shad- 
ow of the earth, as shown 
at C, is circular, indicating 
the globular form of the 
earth. An observer may 
be seen on the upper side 
of the earth, within the 
shadow, looking at the eclipse.] 

27. State the last proof that the earth is a globe. 

It is certain from the fact SHIP9 sai «n« around the world. 
that many ships have sailed 
quite around it, going west- 
ward till they came to the 
point whence they started. 

[Ferdinand Magellan, a Portuguese, was 
the first person who sailed around the 
world. He sailed from Seville, in Spaiu, 
in 1521. It is now quite common for ships 
to go to China by Cape Horn, and return 
by the Cape of Good Hope, thus circum- 
navigating the globe.] 

2§. What proof have we that the earth revolves or turns over 1 
The apparent revolution of the sun, moon, and stars, 
from east to west, every twenty-four hours, shows the 
real revolution of the earth from west to east in the 
same time. 

[1. That the earth revolves upon its axis may be inferred from the fact that a sphere 
is well adapted to such a revolution, and that it is impossible for man to project a ball 
through space without having it revolve on its axis at the same time, 

2. That the heavenly bodies appear to revolve westward, is no proof that they are 
actually in motion. We often transfer our own motion, in imagination, to bodies that 
are at rest, as already shown at Question 9. " Copernicus tells us that he was first led 
to think that the apparent motions of the heavenly bodies, in their diurnal revolution, 
were owing to the real motion of the earth in the opposite direction, from observing 
instances of the same kind among terrestrial objects; as when the shore seems to the 
mariner to recede as he rapidly sails from it; and as trees and other objects seem 




14 PKIMARY ASTRONOMY. 



to glide by us, when, on riding swiftly past them, we lose the consciousness of our 
own motion." 

3. For the sun and fixed stars to revolve daily around the earth would require an 
inconceivable velocity. To perform their respective journeys, the sun would have to 
fly onward at the rate of 25 millions of miles per hour, or 69,440 miles per second ; 
and the nearest fixed stars at the rate of 14,000 millions of miles per second, or 70,000 
times as swift as light ! 

4. The obvious design of the sun is "to rule the day," and give us the agreeable 
vicissitudes of day and night. Now for the sun to go around the earth, to enlighten 
and warm its different sides, would be like carrying the fireplace around a person, in 
order to warm him. It was in view of this absurdity in the Ptolemaic theory that an 
ancient philosopher said that if he had made the universe, he could have made it 
better than the gods had made it. 

5. The whole Copernican system, of which the revolution of the earth is a leading 
principle, is demonstrated to be correct by the prediction of Eclipses and Transits, with 
the greatest possible exactness, and for years before they take place. These calcula- 
tions are all based upon the truth of the Copernican theory, and if the theory was not 
correct the predictions would fail; but as they do not, it follows that the theory upon 
which they are based is correct.] 



LESSON III. 



29. What is a Solid or Body ? 

It is any thing having length, breadth, and thick- 
ness. 

SO. Describe a surface. 

It is the outside or exterior of a body, and has length 
and breadth only. 

31. How are surfaces distinguished? 

Into Plane, Convex, and Concave. 

[A surface may also be rough or smooth, hard or soft, but the question has reference 
to the general figure of bodies, and the answer is given accordingly.] 



32. What is a Plane Surface? 



* As some knowledge of geometrical terms and figures is necessary 
in the study of Astronomy, two lessons upon the subject are here in- 
serted for the benefit of those students who have not studied Geometry. 
An introduction to some of its first principles, in this connection, will 
probably awaken a desire in the minds of such students, to become fur- 
ther acquainted with this beautiful and sublime study. 



PRIMARY ASTRONOMY. 



15 



It is a surface that is perfectly flat, like the top of a 
table or the side of a wall.* 

[1. We may imagine what is called a plane, to extend off beyond the plane surface 
as far as we please ; or, in other words, to be indefinitely extended. When a plane or 
a line is extended in this way, it is said to be produced. 

2. An imaginary plane may exist where there is no body having a plane surface ; or 
between two lines, like the plane of a circle. A sheet of tin, laid across a small wire 
hoop, would represent the plane of that circle, in whatever position it might be held, 
whether horizontally, perpendicularly, or otherwise; and the place which the tin 
would pass through, if extended to the starry heavens, is the plane of that circle. 

3. All objects which the tin would touch or cut, if extended outward to the 
heavens, or to infinity, are in the plane of the sheet, or the circle upon which it is laid. 
A point is in a plane produced, when the plane continued or extended would pass 
through that point.] /i * 



PERPENDICULAR PLANES. 



S3. When are Planes said to be Parallel ? 

When so placed that they would never meet 
or cut each other, however far they might be ex- 
tended. 

[The two sides of a board, or two sheets of tin placed equidistant from 
each other at every point, represent parallel planes.] 

34. When are Planes said to be 
Perpendicular 1 

When one stands exactly up- 
right upon the other ; or when 
they cut each other at right 
angles. 

[In the figure one plane is placed horizontally, 
and the other perpendicular to it. They are 
therefore perpendicular to each other, however 
they may stand in relation to the observer.] 

35. When are Planes said to be In- 
clined 1 

When they intersect or cut 
each other obliquely. 

36. What is meant by the Angle of 
Inclination 1 

It is the angle contained be- 




* A plane surface is to be distinguished from a pic 



16 



PRIMARY ASTRONOMY. 



ANGLE OF INCLINATION. 





A SPHERE. 



tween the two surfaces of the 
planes nearest each other. 

[The spaces A and B in the adjoining cut 
represent the Angle of Inclination.] 

37. What is meant by the Area* of a Plane Figure'} 

It is the amount of surface contained therein. 

3§. What is a Convex Surface ? CONVEX AND C0NCAVE SURFACKS - 

It is one that is swollen out 

like the outside of a bowl. 

39. Describe a Concave Surface. 
It is one that is hollowed out 

like the inside of a bowl. 

40. What is a Sphere ? 
It is a Globe or Ball, every part of the 

surface of which is equidistant from a point 
within called its center. 

[Hence the word spherical, or round, like a sphere.] 

41. What is a Hemisphere 1 

It is the half of a sphere or globe. 

[In Geography we often read of the Eastern and Western, and North- 
ern and Southern hemispheres, but in Astronomy the term is only ap- 
plied to the Northern and Southern portions of the heavens.] 

42. Describe a Spheroid.^ 
It is a body resembling a sphere, but yet not perfectly 

round or spherical. 

43. How many kinds of Spheroids are there ? 
Two : the Oblate and the Oblong. 

44. What is an Oblate Spheroid ? 

It is a globe that is slightly flattened, 
as if pressed on opposite sides. 

[This is a very difficult figure to represent upon paper. 
Should the pupil fail to obtain a correct idea, the Teacher 
will be at no loss for an illustration.] 




HEMISPHERE. 




OBLATE SPHEROID. 




* A'-RE-A. 



f Sphe'-roid. 



PRIMARY ASTRONOMY. 



17 



OBLONG SPHEROID. 




45. Describe an Oblong Spheroid. 
It is an elongated sphere. 

[This figure, like an Oblate Spheroid, admits of various 
degrees of departure from the spherical form. It may be 
much or but slightly elongated, and the ends may be alike 
or otherwise. A common egg is an Oblong Spheroid.] 

46. What is meant by the Axis 
of a Sphere? 

It is the line, real or im- axis ( % 
aginary, around which it re- 
volves. _J3U 

[A wire run through the center of a round apple would represent the axis of a 
sphere.] 

47. What are the Poles of a Sphere. 

The extremities of its axis, or the points where the 
axis cuts the two opposite surfaces. 

4§. Describe the Equator of a Sphere. 

It is an imaginary circle upon its smface, midway 
between its poles, the plane of which cuts the axis per- 
pendicularly, and divides the sphere into two equal 
parts or hemispheres. 

49. By what other name is the Equator of a Sphere sometimes 
designated ?* 

It is sometimes called a Great Circle, because no 
larger circle can be drawn upon a sphere. 

50. What is a Less Circle ? great and less circles. 
It is one that divides a sphere 

into two imequal parts. 



[In the cut the circles are represented in per- 
spective. The Great Circle embraces the middle of 
the sphere, where its full diameter is included ; while 
the Less Circle passes around it between the Equator 
and the Poles, and is consequently " less" than the 
Equator. The subject of the Equator will be further illustrated in Lesson VI., where 
we speak of the Celestial and Terrestrial Spheres.] 




* Des'-ig-xa-ted. 



2* 



18 



PEIMAEY ASTEONOMY. 



51. What are the Meridians of a Sphere ? 

They are lines drawn from pole to 
pole upon its surface. 

[Meridians all meet at the Poles, and are most distant at 
the Equator, as shown in the cut. Hence the length of a 
degree of longitude depends entirely upon the latitude of the 
place where it is measured. It varies from 69£ miles to 
nothing.] 




LESSON IY. 

OF LINES AND ANGLES.* 

52. What is a Point? 

It is that which has no magnitude or extension, but 
simply position. 

[" The common notion of a point is derived from the extremity of some slender 
body, such as the extremity of a common sewing-needle. This being perceptible 
to the senses, is a physical point, and not a mathematical point ; for, by the definition, 
a point has no magnitude." — Professor Perkins.] 

53. What is a Straight or Right Line 1 

It is the shortest distance between two 
points. 

54. What is a Curve Line ? 
It is one that departs continually from 

a direct course. 

55. What are Parallel Lines ? 
They are such as remain at the same 

distance from each other throughout their 
whole extent. 

56. What are Oblique Lines ? 
Such as are not parallel, but incline to- 
ward or approach each other. 



A RIGHT LINE. 



CURVE LIN] 



PARALLEL LINES. 



OBLIQUE LINES. 



* The judicious Teacher will find it advantageous to make a free 
use of the Blackboard during this and the two following lessons. 



PEEMAEY ASTKOXOMY. 19 



AN ANGLE. 



57. When two lines intersect each other, what is 
the space included between them called ? 

An Angle. 

58. How are Angles divided? 

Into three kinds ; namely, the Might Angle, the 
Acute, and the Obtuse. 

59. What is a Right Angle ? 

It is one formed when one straight line R1SHT ASOLE8 . 
intersects another perpendicularly. The | 

angles on each side are equal, and are 
Eight Angles. - ' 

60. What are Acute and Obtuse Angles 1 acute and obtuse 

. ANGLES. 

w hen one right line intersects another 
obliquely, the smaller angle is called an 
acute angle, and the larger an obtuse 

analp LARGE ACUTE ANGLE. 

[An acute angle is always less, and an obtuse angle always 
greater, than a right angle. The former may include nearly 
one quarter, and the latter nearly one half of a circle. In 
the first of the adjoining figures the angles vary so slightly from small acute angle. 
right angles, that the obtusencss of the one and the acuteness of 
the other are hardly perceptible. In the other figure one angle 
is very obtuse, and the other very acute. So both these angles 
may vary to the amount of a quarter of a circle.] /acute^ obtuse \ 

61. What is a Triangle ? 

It is a plane figure, bounded by straight lines, and 
having only three sides. 

62. How many kinds of Triangles are there ? 

Four : the Equilateral, Isosceles* Sca- 
lene,-^ and Bight-Angled. 

63. What is an Equilateral Triangle ? 
It is one having three sides equal. 

[Equilateral, from the Latin aequus, equal, and lateralis, from 
latus, side. 

* I-SOS'-CE-LES. f SCA- 



ACUTE \ OBTUSE 




AN EQUILATERAL 
TRIANGLE. 




20 



PKIMARY ASTKOl^OMY. 



64. What is an Isosceles Triangle ? 
It is one having but two sides equal. 

[The term Isosceles is from a Greek word, signifying equal 
legs ; hence a triangle with two equal legs is called an Isosceles 
Triangle. Let the student carefully observe the derivation of 
these words, and he will never forget their meaning.] 

65. Describe a Scalene Triangle. 
It is one having no two sides equal. 

[The term Scalene is from the Greek skalenos, and signifies 
oblique, unequal.] 

66. What is a Right- Angled Triangle 1 
It is one having a right angle. 

67. What are its different sides called respect- 
ively ? 

The Base, the Perpendicular, and the 
HyjootTienitse.* 



ISOSCELES TRIANGLE. 



A 



SCALENE TRIANGLE. 




RIGHT-ANGLED 
TRIANGLE. 




LESSON Y 



OF THE CIRCLE AND THE ELLIPSE. 

68. What is a Circle ? 

It is a plane figure, bounded by a curve 
line, every part of which is equally dis- 
tant from a point within called the cen- 
ter. 

69. What are Concentric Circles ? 
Such as are drawn around a common 

center. 

[If they are in the same plane they must be drawn at different 
distances, otherwise they could not constitute two distinct cir- 
cles. If thus drawn they are parallel, otherwise they are not.] 




CONCENTRIC CIRCLES 



* Hy-poth'-e-nuse, from a Greek word, which signifies to subtend or 
stretch : a line subtended (see 84) from the base to the perpendicular. 



PKBtAKY ASTKONOMY. 



21 



CONCENTRIC CIRCLES IN 
DIFFERENT PLANES. 



70. Must all Concentric Circles be in the 
same plane ? 

No : if drawn around the same 
common center they will be concen- 
tric^ though they lie in different 
planes. 





[Here the three circles are in different planes, but at the same distance. They are 
concentric, but not parallel.] 

71. What is the Circumference of a Cir- diameter, circumfer- 

ence, ETC. 
del 

The curve line which bounds it. 

72. What is the Diameter ? 
It is a right line passing through 

the center, and terminating each way 
in the circumference. 

73. Describe the Radius of a Circle. 

It is a right line drawn from its center to any point 
in the circumference. 

[The plural of radius is radii ; and as radii proceed from a common center, light, 
which proceeds from a luminous point in all directions, is said to radiate ; and the 
light thus dispersed is sometimes called radiations or radiance.] 

74. How is the Circumference of a Circle divided ? 
Into Signs, Degrees, Minutes, and Seconds. 

[The student will probably recognize these divisions as what he has previously 
learned among his Arithmetical Tables, under the head of " Circular Measure."] 

75. What is a Sign ? 

The twelfth part of a circle. 

76. What is a Degree ? 

The 30th pcvrt of a sign, or the 
360th part of a circle. 

77. What is a Minute? 
The 60th part of a degree, or the 

12,600th part of a circle. 
7§. What is a Second? 



PARTS OF A CIRCLE. 





22 PRIMARY ASTRONOMY. 

The 60tli part of a minute, or the 756,000th part of a 
circle. 

[1. Thus all circles, whether great or small, are supposed to be divided into 360 
equal parts called degrees, and marked thus, 360°. Each degree is again subdivided 
into 60 equal parts, called minutes, and each minute into 60 equal parts, called 
seconds. The minutes are marked thus, 60', and the seconds thus, 60". 

2. To save the trouble of dividing a cir- A protractor. 

cle into 360°, in order to measure the 
degrees of an angle, we make use of an 
instrument called a Protractor, It consists 
of a semicircle of silver or brass, divided 
into degrees, as represented in the inclosed 
figure. To measure an angle, as A, B, C, iRp^ 
the straight edge of the protractor is placed ftaj® 
upon the line B C, so that the center 
around which it is drawn will be exactly 
at the intersection of the lines, or point of the angle, as at B ; then the number of de- 
grees included between the lines on the protractor will represent the quantity or 
amount of the angle. From this it will be seen that the amount of the angle does not 
depend upon the length of the lines which form it, nor upon the magnitude of the 
circle on which the degrees are marked by which it is measured, but simply upon the 
width of the opening between the lines, as compared with the whole circumference 
around the point B. A circle marked off into degrees, minutes, and seconds, is called 
& graduated circle^ 

TO. What other parts of a Circle can you mention ? 

The Semicircle, Quadrant, Sextant, and Arc. 

§©. What is a Semicircle ? 

It is one half of a circle, and contains 180°. 

§1. What is a Quadrant 1 

It is the fourth part of a circle, and contains 90°. 

[The term Quadrant is applied to a nautical instrument, of the form of a quarter of a 
circle, which is much used by navigators in determining the altitude or apparent 
height of the sun, moon, and stars.] 

§2. Describe the Sextant. 

It is the sixth part of a circle, and contains only 60°. 

[The word Sextant also denotes an instrument similar to a Quadrant, and used for 
similar purposes. The main difference is, that one represents 60° and the other 90° 
of a circle. The Octant, or eighth part of a circle, is also used for similar purposes.] 



§3. What is an Arc* of a Circle 1 

It is any part of it less than a whole. 



* From the Latin arcus, a bow, vault, or arch. By associating the 
word arc with arch, the student will always remember its meaning. 



PKIMAKY ASTRONOMY. 



23 




§4. What is a Chord ? ARC AND CH0RD - 

It is a straight line within a circle, 
joining the extremities of an arc. 

[The Chord of an Arc is said to be subtended (from sub, 
under, and teno, to stretch), because it seems stretched under 
the arc like the string of a bow. In the cut there are four 
arcs and as many chords. The lower arc is a large one, 
while the arc and chord, A C, are quite small. Still each 
division of the circle, whether great or small, is an arc, and the line joining the ex- 
tremities of each arc, respectively, is a chord.'} 

§5. What is an Ellipse ? an ellipse. 

It is an oblong figure, like an 

oblique view of a circle. 

§6. In what respect does it differ from 
a Circle'} 

Its diameters are unequal ; and 
it has two points called its Foci,* 
around which, as centers, the 
figure is described. 

§7. How are the unequal diameters of an Ellipse distinguished ? 

The longer is called its Major MAJ0R AND MIN0R AXES -+ 
and the shorter its Minoi' % Axis. 

[The longer is sometimes called the Transverse 
and the shorter the Conjugate Axis, but major and 
minor are more simple and perspicuous, and there- 
fore preferable.] 

§§. What is meant by the Eccen- 
tricityX of an Ellipse ? 

It is the distance between 
the center and either focus. 

[All circles drawn around two different points 
as centers are called eccentric circles.] 





* Fo'-ci is the plural of Fo-cus. f Ax'-es is the plural of Ax-is. 

\ Ec-cen'-tric, ex, from, and centrum, center. Hence a circle that 
varies in its distance from the center is eccentric. So, also, persons who 
depart from the usual round of thought and custom are called eccentric 
persons. 



24 PEIMAEY ASTRONOMY. 

LESSON yi. 

THE TERRESTRIAL AND CELESTIAL SPHERES. 

§9. Can you repeat the definition of a Sphere given in a pre- 
ceding lesson ? (Question 40.) 

90. What then do you understand by a Terrestrial* Sphere! 
The Earth or Globe which we inhabit. 

[Though the earth is not strictly speaking a sphere, as that figure is defined at 41, 
but rather an oblate spheroid (44), still it is usually called a sphere, on account of its 
near approach to that figure, and as a matter of convenience. A common artificial 
globe is a good representation of the terrestrial sphere.] 

91. What is the Axis of the Earth ? 

The imaginary line about which it revolves (46). 

92. What are the Poles of the Earth ? 

The extremities of her axis where they cut or pass 
through the earth's surface (47). 

[The wire upon which an artificial globe turns represents the earth's axis, and the 
extremities the North and South Poles.] 

93. What is the Equator of the Earth ? 

An imaginary circle drawn around it, from east to 
west, at an equal distance from each Pole, and dividing 
it into two equal parts, called Hemispheres (48). 

[See definition and illustration, page 17.] 

94. Wliat is Latitude ? 

Distance ISTorth or South of the Equator. 

95. How is it reckoned ? 

From the Equator each way, in Degrees, Minutes, 
and Seconds. 

[As the distance from the Equator to the Pole cannot be more than a quarter of a 
circle, or 90°, it is obvious that no place can have more than 90° of latitude ; or, in 
other words, all places upon the earth's surface must be between the Equator and 90° 
of latitude, either north or south.] 

* Tek-hes'-tri-al, from terre, the earth : pertaining to the earth. 
Hence St. Paul says, 1 Cor. xv. 40, " There are also celestial bodies, and 
bodies terrestrial ; but the glory of the celestial is one, and the glory of 
the terrestrial is another. 



PEDIART ASTEOXOMY. 



25 



PARALLELS. 




THE TROPICS AND POLAR 








CIRCLE. 










E^ 






E 






F/ 




\ DC 






V 




Af 




: o 
1- 


AX MS 






A 






< 










\ 


/ 


. Z) 










\ / 




.' Q 










\ 


v / 


Ell 











96. What are Parallels of Latitude ? 
Circles either Xorth or South, of the 

Equator, and running parallel to it. 

[We may imagine any conceivable number of parallels 
between tbe Equator and the Poles, though upon most 
maps and globes they are drawn only once for every ten 
degrees.] 

97. What are the Tropics 1 

Two parallels of latitude, each 
23° 28' from the Equator. 

98. What are they called respectively? 
The Southern is called the Tro- 

jnc of Cancer, and the Northern 
the Tropic of Capricorn. 

[1. The Tropical Circles are shown at E E in the an- 
nexed figure. 

2. The sun never shines perpendicularly upon any points on the earth further from 
the Equator than the Tropics. Between these he seems to travel regularly, leaving 
the Tropic of Cancer on the 23d of December, crossing the Equator northward on the 
20th of March, reaching the Tropic of Capricorn on the 21st of June, crossing the 
Equator southward on the 23d of September, and reaching the Southern Tropic 
again on the 23d of December. In this manner he seems to cross and recross the 
Equator, and vibrate between the Tropics from year to year. The cause of this ap- 
parent motion of the sun will be explained in a subsequent lesson.] 

99. What are the Polar Circles ? 

They are two parallels of latitude, 23° 28' from the 
Poles. (See cut.) 

100. What are tliey called respectively ? 

The Northern is called the Arctic, and the Southern 
the Antarctic, Circle. 

[These circles are shown at F F in the preceding figure.] 

101. How do the Tropics and Polar Circles divide the surface of 
the Globe J 

Into five parts, called Zones. 

[A zone properly signifies a girdle ; but the term is here used in an accommodated 
sense, as only three of these five divisions at all resemble a girdle. The parts cut off" 
by the polar circles are mere convex segments of the earth's surface.] 



102. How are these Zones classified 



26 



PKIMAEY ASTKO]SOMY. 



Into Torrid,* Temperate, and Frigid.\ 

103. How many of each ? 

One Torrid, two Temperate, and two Frigid 

104. Where are they located, respectively ? 
The torrid, between the Tropics / the 

temperate, between the Tropics and 



THE FIVE ZONES. 




MERIDIANS. 




the Polar Circles ; and the frigid, be- 
tween the Polar Circles and the 

Poles. 

105. What are Meridians ? 
Imaginary lines drawn from Pole 

to Pole over the earth's surface. 

[Meridians cross the Equator at right angles; and the 
plane of any two Meridians directly opposite each other 
would divide the earth into Eastern and Western Hemi- 
spheres, aa the Equator divides it into Northern and South - 
em. We may imagine Meridians to pass through every 
conceivable point upon the earth's surface. They meet, at the Poles, and are furthest 
apart at the Equator.] 

106. What is Longitude 1 

It is distance East or West from any given Meridian. 

107. How is it reckoned ? 

Both East and West, in Degrees, Minutes, and 

Seconds. 

[A degree of longitude at the Equator comprises about (59J miles, but is less and 
less as the meridians approach the Poles, at which points it is nothing. A degree of 
latitude is about 69£ miles on all parts of the globe.] 

10§. What is the Meridian called from which we commence to 

reckon Longitude 1 

The First Meridian. 

[On European charts and globes longitude is usually reckoned from the Royal Ob- 
servatory at Greenwich, near London; but in this country it is often reckoned from 
the Meridian of Washington. It would be better for science, however, if all nations 
reckoned longitude from the same Meridian, and all charts and globes were con- 
structed accordingly.] 



* Tor ''-rid, parched, dried with heat, 
f Frig '-id, cold, wanting heat or warmth. 



PRIMARY ASTRONOMY. 



m 



109. What is the greatest Longitude that any place can have ? 
One hundred and eighty degrees (180°). 

110. What is meant by the Sensible Horizon 1 

It is that circle which terminates our view, or where 
the earth and sky seem to meet. 

111. What is the Rational Horizon ? 

It is an imaginary plane, below the visible horizon 
and parallel to it, which, passing through the earth's 
center, divides it into upper and lower hemispheres. 

[1. These hemispheres are distinguished as upper 
and lower with reference to the observer only. 

2. The sensible horizon is half the diameter of the 
earth, or about 4000 miles from the rational : and 
yet so distant are the stars that both these planes 
seem to cut the celestial arch at the same point; 
and we see the same hemisphere of stars above the 
sensible horizon of any place that we shoidd if the 
upper half of the earth were removed, and we stood 
on the rational horizon of that place.] 



-&-H0RI 




112. What are the Zenith and Nadir Points ? 
The Zenith is the point directly overhead, and the 
Nadir the point directly under our feet. 

[1. These directions, it must be remembered, are merely relative. As the earth is a 
sphere, inhabited on all sides, the Zenith point is merely opposite its center, and the 
Nadir toward its center. So with the directions Up and Down : one is from the 
center, and the other toward it ; and the same direction which is up to one is down to 
another. This fact should not merely be acknowledged, but should be dwelt upon 
until the mind has become familiarized to the conception of it, and divested, as far 
as possible, of the notion of an absolute up and down in space. We should remem- 
ber that we are bound to the earth's surface by 
attraction, as so many needles would be bound to 
the surface of a spherical loadstone. 

2. East and West also are not absolute, but 
merely relative directions. East is that direction 
in which the sun appears to rise, and West is 
the opposite direction ; and yet, so far as absolute 
direction is concerned, what is East to one, as 
to the observer at A, is West to B, and so with 
C and D. And as the earth revolves upon its 
axis every twenty-four hours, it is obvious that 
East and West upon its surface must, in that 
time, change to every point in the whole circle 
of the heavens. The same is true of the Zenith 
and Nadir, or of up and down.] 



UP AND DOWN, AND EAST AND 
WEST. 

A 

- OS EAST HE 




28 



PKIMAEY ASTEONOMY. 



113. What is meant by Space in Astronomy ? 

It is the interval or void between the earth and the 
heavenly bodies, and extending onward beyond them 
all in every direction. 

[Space has no limits, or, in other words, is boundless or infinite. Suppose six 
persons were to start from as many different points upon the earth's surface, as, for 
instance, one from each pole, and one from each of the positions occupied by observers 
in the next figure. Let them ascend or diverge from the earth in straight lines, 
perpendicularly to its surface ; and though they were to proceed onward, separating 
from each other with the speed of lightning for millions of ages, none of these 
celestial voyagers would find an end to space, or any effectual barrier to hinder 
their advancement. Should they chance to meet another world in the line of their 
flight, it would soon be passed, like a ship met by a mariner upon the ocean, and 
beyond it space would still invite them onward to explore its immeasurable depths. 
And thus they might go on forever, without changing their position in respect to 
the center or boundaries of immensity ; for as eternity has no beginning, middle, or 
end ; so space is without center or circumference, an ethereal ocean, without bot- 
tom or shore.] 



TERRKSTRIAL AND CELESTIAL 
SPHERES. 




114. What is the Celestial Sphere ? 
It is the apparently concave 

surface of the heavens, in which 
the stars seem to be set, and 
which surrounds the globe on 
every side. 

[The relation of the Terrestrial to the Celes- 
tial Sphere may be understood by the annexed 
diagram, in which the stars surround the earth 
in all directions, as they seem to fill the whole 
celestial vault.] 

115. What is the Equinoctial ? 
It is the Celestial Equator, or the 

plane of the earth's equator extend- 
ed in every direction to the starry 
heavens. 

116. What is Declination ? 
It is apparent distance either north 

or south of the Equinoctial. 

[Declination is the same to the heavens that latitude is to the earth.] 

117. What is Right Ascension ? 



THE EQINOCTIAL. 




PRIMARY ASTRONOMY. 29 

It is distance east of a given point, and is reckoned 
on the Equinoctial quite around the heavens. 

[hi one respect Right Ascension in the heavens is like longitude on the earth : 
they are both reckoned upon the equators of their respective spheres ; but while 
longitude is reckoned both east and west of the first meridian, and can only amount to 
180°, Right Ascension is reckoned only eastward, and consequently may amount to 
360°, or the whole circle of the heavens. The principal difference between Right As- 
cension and Celestial Longitude is, that the former is reckoned on the Equinoctial, 
and the latter on the Ecliptic] 



FIRST GRAND DIVISIONS OF THE UNIVERSE. 

118. What does the term Universe signify ? 

The whole system of creation ; or every thing visible 
and invisible in heaven and earth. 

119. How is the Universe divided? 
Into Matter and Spirit 

120. What do you understand by Matter ? 

It is any thing that you can see or feel, or that has 
length, breadth, and thickness. 

[Of the essence or elements of matter we know nothing, our knowledge being con- 
fined entirely to its properties.'] 

121. What is Spirit? 

It is that which learns, thinks, and reasons. It is the 
same as the intellect, mind, or soul. 

[As with matter, so with spirit; we know it only by its properties or qualities.] 

122. How is Matter divided? 
Into Animate and Inanimate. 

123. Describe each. 

Inanimate matter has no feeling or sensation, like 
stones and trees ; but animated matter has animal life, 
like the bodies of living men and beasts. 

124. To which division do the Heavenly Bodies belong ? 

To that of inanimate matter. 



3* 



30 



PRIMARY ASTRONOMY. 




SOLAR SYSTEM AND SIDEREAL HEAVENS. 



125. What are the first THE SOIAR system. 
grand divisions of the Heav- 
enly Bodies ? 

The Solar System 
and the Sidereal 
Heavens. 

126. What constitutes 
the Solar System ? 

It includes the sun, 
and all the worlds that 
revolve around him. 

[This system of worlds derives 
its name from the Latin term Sol, 
the Sun ; hence the Solar System signifies the System of the Sun.'] 

127. What do the Sidereal Heavens include ? 

All those bodies that 
lie arotmd and oeyond 
the Solar System, in 
the region of the Fixed 

Stars. 

[1. The word Sidereal is from the 
Latin sidcralis, and signifies per- 
taining- to the stars. The Sidereal 
Heavens are, therefore, the heavens 
of the fixed stars. 

2. The relation of the Solar Sys- 
tem to the Sidereal Heavens is 
shown in the annexed cut, where 
the sun appears only as a star at a 
distance from all others, and sur- 
rounded by his own retinue of 
worlds. The Solar System is drawn 
upon a small scale, and the Sidereal Heavens are seen around and at a distance from 
it in every direction.] 

12§. What part of the book have you now gone over ? 
Part First, which consists of Preliminary Observa- 
tions and Definitions. 

129. What do you enter upon in your next Lesson ? 
Part Second, which treats of the Solar System. 




PRIMARY ASTRONOMY. 31 

PART II. 
OF THE SOLAR SYSTEM. 



LESSON VII. 

BODIES THAT COMPOSE THE SYSTEM. 

130. Of what bodies does the Solar System consist? 

Of three classes; namely, the Sun, Planets, and 

Comets, 

131. How is the Sun distinguished 1 

As the fixed center of the system, around which the 
other bodies revolve ; and as the largest and only self- 
luminous body in the system. 

132. Describe the Planets. 

They are those large globes or worlds that revolve 
around the sun, and receive their light and heat from 
him. 

[The terra Planet signifies a wanderer ; and is applied to some of the solar bodies, 
because they seem to wander or move about among the stars.] 

133. How are the Planets divided? 

Into Primary and Secondary, 

134. What are the Primary Planets? 

Such as revolve around the sun only, as their center 
of motion. 

135. What are the Secondaries? 

They are small planets that revolve around the 
Primaries, and accompany them in their revolution 
around the sun. 

[The Secondary Planets may be seen near their respective Primaries in the cut, 
page 9- They are often called Moons or Satellites. A satellite is a follower or at- 
tendant upon another.] 



32 PEIMAET ASTEONOMY. 

136. What is the Orbit of a Planet ? 

It is the path it pursues in its revolution around the 
sun. 

[The Orbits of the planets are represented by the white circles in the cut above re- 
ferred to.] 

137. What do you understand by the Interior and Exterior 
Planets ? 

The Interior are those whose orbits lie within, and 
the Exterior those whose orbits lie without the orbit of 
the earth. 

[Some Astronomers speak of these two classes, respectively, as Inferior and Superior. 
The reason seems to be, that as those nearer the sun than the earth are lower than she 
is — that is, nearer the great center of the system — they are, in this respect, inferior 
to her ; while, on the other hand, those that are above or beyond her, are her superiors. 
But as the distinction is founded upon, and is intended to denote the position of the 
planets with respect to the earth's orbit, it is obvious that interior and exterior are the 
more appropriate terms. It seems hardly allowable to call the Asteroids superior 
planets, and Mercury and Venus, which are much larger, inferior.] 

138. What are the Asteroids 1 

They are the twelve small planets revolving between 
the orbits of Mars and Jupiter. 

[Jlsteroid signifies star4ike, and is applied to these small planets l>ecause of their 
comparative minuteness. They are never seen except through telescopes, and through 
ordinary instruments are not always readily distinguished from the fixed stars.] 

139. What are Comets ? 

They are a class of bodies distinguished for their long 
trains of light, their various shapes, and the great ec- 
centricity (93) of their orbits. 

[A Comet and part of its orbit are shown in the upper cut, page 30, to which the 
student is referred. In the miniature representation of the Solar System, on the same 
page, the whole of a Comet's orbit is exhibited.] 

140. Are the Planets and Comets self-luminous, or do they shine 
merely by reflection 1 

They are all opake* bodies, and shine only as they 
are illuminated by the sun. 

[That the planets and comets are opake, is obvious from the fact that the side 
toward the sun is all that ever looks bright, as is seen in the case of the new moon. 

* 0-pake', dark, obscure. 



PRIMAKY ASTRONOMY. 33 



Hence the various phases or appearances of the planets. Again : whenever, by any 
means, the light of the sun is intercepted or cut off, the planet, thus deprived of its 
borrowed rays, ceases to shine. Hence what are called Eclipses of the Moon.] 



LESSON VIII. 

NAMES OF THE PLANETS, SIGNS, ETC. 

141. How many Primary Planets are there 7 
Twenty. 

[Nine of these have been discovered within a few years, and it is not improbable 
that there are several others, of the family of the Asteroids, that will hereafter be dis- 
covered. We speak of twenty as the number now known.] 

142. What are the Names of the Primary Planets ? 
Beginning at the sun, and passing outward, they 

are — 



Mercury. £. 

Venus 9- 

Earth 0. 

Mars g. 

Flora 

Vesta. g. 

Iris 

Metes 

Hebe e=> p 

Hygeia 



Astr^ea 

Juno §. 

Parthenope.* M 

Cuof 

Ceres ^>. 

Pallas $. 

Jupiter 2_f . 

Saturn Tj>. 

Herschel and IH. 

Neptune:}: T£ # 



[It is important for the student to commit these names to memory in the order in 
which they here occur, as it will help to fix in his mind the relative positions of the 
planets, and greatly facilitate the acquisition of further knowledge respecting them.] 

143. After whom are the Planets named? 
After heathen gods and goddesses. 

144. Why is this? 

Because Astronomy was first studied by Pagan 

* Par-then r -o-PE, the ninth Asteroid, discovered May 11, 1850. See 
definition of Asteroids, Question 138. 

f The Asteriod discovered Sept. 13, 1850. 

% This planet was first called Le Verrier, but is now more generally 
known by the name of Neptune. 



34 



PRIMARY ASTRONOMY. 



nations, who named the planets then known after their 
imaginary divinities. 

[A history of these fabulous beings is what is called Mythology.'] 

145. Who was Mercury, in Mythology ? 

He was the messenger of the gods, and the patron of 
thieves and dishonest persons. 

146. What does his Astronomical Sign signify? 

It denotes his caduceus or rod, with serpents twined 
around it ( S ) * 

[1. Mercury was represented as very eloquent, and skillful in interpreting and ex- 
plaining—as the god of rhetoricians and orators. Hence, when Paul and Barnabus 
visited Lystria, addressed the people, and wrought a miracle, they said, "The gods have 
come down to us in the likeness of men. And they called Barna- R0D QF MERCURY . 
bus Jupiter, and Paul Mercurius, because he was the chief speaker." 
See Acts xiv. 8-13. 

2. " The caduceus of Mercury was a sort of wand or scepter, 
borne by Mercury as an ensign of quality and office. (Jn medals 
it is a symbol of good conduct, peace, and prosperity. The rod 
represents power; the serpents, wisdom; and the two wings, 
diligence and activity.'''' — Encyclopaedia. 

3. The original form of this sign may be understood by the 
annexed cut, to which the present astronomical symbol ( £ ) 
bears but a slight resemblance.] 

147. Who was Venus \ 




The Goddess of Beauty and Love. 

148. What is her Sign? 

It is a Mirror or Looking-glass, which 
she is represented as carrying in her 
hand (9). 

[Anciently mirrors were made of brass or silver, highly 
polished, so as to reflect the image of whatever was brought 
before them. Hence it is said in the Book of Exodus, 
written fifteen centuries before Christ, that Moses "made 
the laver of brass, and the foot of it of brass, of the looking' 
glasses of the women," &c. For convenience, the ancient 
mirrors had a handle attached, as represented in the cut, 
which very much resembles the sign of the planet.] 



MIRROR OF VENUS. 




* All these symbols should be drawn in rotation upon the Blackboard 
during recitation, by the Teacher or some member of the class. It will 
be well, therefore, for the student to observe each sign carefully, that he 
may be prepared to draw and explain it if called upon. 



SPEAR AND SHIELD 
OF MARS. 




PRIMARY ASTRONOMY. 35 

149. What Sig?i represents the Earth? 

She lias two ; one representing a sphere and its equa- 
tor (0), and the other (©) the four quarters of the globe. 

150. Describe Mars and his Sign. 

Mars was the God of War, and his sign 
(<?) represents an ancient shield or buck- 
ler, crossed by a spear. 

[Gunpowder was not known to the ancients, so they had no 
pistols, muskegs, or cannon. They fought with short swords and 
| spears, and defended themselves with the shield, carried on the 
left arm. A shield aud spear were, therefore, very appropriate 
emblems of war. The original form of the sign of Mars is 
presented in the cut.] 

151. Who was Flora 1 

The " Queen of all the Flowers." 

152. Describe Vesta and her Sign. 

She was the Goddess of Fire,&n(i her sign is an 
altar (£), with &Jire blazing upon it. 

153. Who was Iris ? 

The beautiful waiting-maid of Juno. Her sign ( ) 
is a rainbow. 

[It was the special office of Iris to stir up strife and discord among men, and to 
separate the soul and body of the dying.] 

154. Who was Metes ? 

The first wife of Jupiter, and the Goddess of Pru- 
dence and Sagacity. 

155. Describe Hebe and her Sign. 

Hebe presided over children and youth, and was 
cup-bearer to Jupiter. Her sign ( ^ ) is a cup. 

[Hebe was celebrated for her beauty ; but happening one day to stumble and spill 
the nectar, as she was serving Jupiter, she was turned into an ostler, and doomed to 
harness and drive the peacocks of the queen of heaven.] 

156. Who was Astr^ea, and what is her Sign 1 

She was the Goddess of Justice, with the sign of a 
balance ( ). 




36 PRIMARY ASTRONOMY. 

[1. Mythology taught that Justice left heaven, during the golden age, to reside on 
the earth ; but, becoming weary with the iniquities of men, she returned to heaven, 
and commenced a constellation of stars. Virgo and Libra in the zodiac still represent 
the goddess Astraea and her golden scales. 

2. The female figure, holding a pair of scales, in the coat of arms of several of the 
United States, is a representation of Astraea, and denotes Jv£tice.~] 

157. Describe Juno and her Sign. 

She was the reputed Queen of Heaven; and her 
sign ( § ) is an ancient mirror , crowned with a star — an 
emblem of beauty and pow6r. 

15§. Describe Ceres and her Sign. sickle or ceres. 

She was the Goddess of Grain and 
Harvest, and her sign (?) is a sickle. 

159. Describe Pallas and her Sign. 

She was the Goddess of Wisdom and 
of War. She was represented as carrying a spe<w, 
which she brandished* terribly in time of battle ; hence 
her sign ($) is the faad of a spear. 

160. Who was Hygeia ? 

The Goddess of Health, and the daughter of Escula- 
pius, the father of the healing art. 

[Our modern word Hygeian, which signifies the laws of health, &a, is derived from 
the goddess Hygeia.] 

161. Who was Parthenope 1 

She was one of the three Syrens — a sea nymphf of 
rare beauty. They were all admirable singers ; hence a 
lyre% (©) is her appropriate sign. 

[1. The three Syrens, Parthenope, Ligeia, and Leucosia, were represented as dwell- 
ing upon the coast of Sicily, and luring mariners upon the rocks of destruction by their 
enchanting songs. Hence whatever tends to entice or seduce to ruin, is often called a 
" syren song." 

2. As this planet wa3 discovered at the Naples Observatory, in Italy, it was quite 
appropriate to name it after one of the Syrens, that Mythology located on the coast of 
a neighboring island.] 



* Brand'-ish, to wave, shake, or flourish. 

\ Nymph, a youthful goddess, inhabiting some particular locality. 

\ Lyre, a harp. 



PRIMAKY ASTRONOMY. 



37 



OLD SATURN, OR CHRONOS. 



162. After whom is Clio, the last discovered Asteroid, named, and 
what is her Sign 1 

After Clio, one of the Muses. Her sign is a star, 
with a sprig of lanrel over it ( ). 

163. Who was Jupiter ? 

He was the reputed father of the gods, and the King 
of Heaven. 

164. What does his Sign (U) represent ? 

It was originally the Greek letter ?, zeta — the same 
as our Z— the initial of the Greek word zeus, the name 
for Jupiter. 

J.65. Describe Saturn and his 
Sign. 

Saturn, called by the 
Greeks Chronos, presided 
over time and chronology. 
He is represented as an old 
man, with wings, bald ex- 
cepting a forelock, with a 
scythe in one hand and an 
hour-glass in the other. His 
sign ( f?) represents a scythe. 

{Chro-nol'-o-gy (from chronos, time, and logos, discourse) ; the science or method of 
computing time, keeping dates, Sec] 

166. How many names has Herschel ? 

Three. 

167. What are they ! 

Georgium Sidus, Uranus, and Herschel. 
16§. Why is it called Georgium Sidus % 
It was so called by its discoverer, in honor of George 
HI. of England, who patronized or assisted him. 
169. What does Uranus signify ? 
It is from Urania, the name of a heathen goddess, 




38 PEIMAEY ASTRONOMY. 

who was represented as presiding over Astronomy, or 
the study of the heavens. 

[From this we have our word uranography, which signifies a description of the 

heavens.] 

170. Why is this planet ever called Herschel ? 

In honor of Sir John Herschel, its discoverer. 

[Notwithstanding the importance of a uniform analogy in naming the planets, this 
one is more popularly known by the name of Herschel than by any other.] 

171. Wliat, then, is the meaning of his Sign? 

It consists of the letter H, with a planet suspended 
from the cross-bar (ijr), to indicate that Dr. Herschel 
was its discoverer. 

172. Describe Neptune and his Sign. 

Neptune was the god of the Seas, but the astronomi- 
cal sign is composed of an L and a Y united, with a 
planet suspended from the hair-line of the Y (^) to in- 
dicate that Le Yerrier was its discoverer. 

173. What Sign is generally used for the Moon? 

She is known by various representations, according 
as she is new, half-grown, or full ; thus, $, d), q. 

174. What is the astronomical emblem of the Sun 1 
It is a shield or buckler — 0, ©, 0. 

[As the ancients often kept their bucklers bright, so as to dazzle the eyes of their 
enemies, this instrument was selected as an appropriate emblem of the Sun.] 



LESSON IX. 

DISTANCES OF THE PLANETS FROM THE SUN. 

175. Will you give the distances of the several Planets, in miles, 
commencing at the Sun ?* 

* Where circumstances permit, and the Teacher is favorable to such 
exercises, these statistics may be learned to good advantage by con- 
cert recitations. 



PEIMAKY ASTRONOMY. 



Mercury 37 millions. 

Venus 69 " 

The Earth. 95 " 

Mars 145 " 

Asteroids, from 225 to 275 " 

[1. These distances, like most of the statistics throughout the book, are given in 
rouud numbers. The design is to impart a tolerably correct general idea, without 
overtasking the memory. The following cut will exhibit to the eye the 

COMPARATIVE DISTANCES OF THE PLANETS, 



Jupiter 495 millions. 

Saturn 900 " 

Herschel 1,800 " 

Le Vender or Nep- ^ 850 
tune 



■w"'.v.-E\ 






rnijrti — ? -f — 

£%/// / I I 

2. To assist his conception of these vast distances, the student may imagine a rail- 
road laid down from the sun to the orbit of Neptune. Now if the train proceed from 
the sun at the rate of thirty miles an hour, without intermission, it will reach Mercury 
in 152 years; the Earth in 361 years; Jupiter in 1,884 years; Saturn in 3,493 years; 
Herschel in 6,933; and Neptune in 10,800 years! Such a journey would be equal 
to riding 900,000 times across the continent, from Boston to Oregon ! 

3. It is now about 5,850 years since the creation of the world. Had a train of cars 
started from the sun at that time toward the orbit of Neptune, and traveled day and 
night ever since, it would still be 284 millions of miles within the orbit of Herschel— 
about where the head of the locomotive stands, as shown in the cut ! To reach even 
that planet would require over 1,000 years longer, and to arrive at Neptune nearly 
6,000 years to come !] 

176. What effect tvould it have upon the apparent magnitude of 
the Sun if we were to go to Mercury 1 

He would appear much larger. 

177. Suppose we were to go out to Herschel or Neptune? 
He would appear much smaller. 

178. Why is this ? 

Because the apparent magnitude of objects depends 
much upon the distance from which they are viewed. 

[1. This may be illustrated by the following cut, representing 

NEAR AND REMOTE VIEWS OF THE SAME OBJECT. 

^£ ;; / / 



Let A represent the position of an observer upon the earth, to whom the Sun appears 
32*, or about half a degree in diameter. Now it is obvious that if the observer advance 




40 PRIMARY ASTRONOMY. 



to B (halfway), the object will fill an angle in his eye twice as large as it filled when 
viewed from A. Again : if he recede from A to C, the object will appear but half as 
large. Hence the rule, that the apparent magnitude is increased as the distance is 
diminished ; and diminished as the distance is increased. 

2. These principles are still further illustrated in the following cut, in which the 
observer is placed at three points, corresponding with the comparative distances of 
Saturn, Herschel, and Neptune. 

THE SUN VIEWED FROM DIFFERENT POINTS. 

4 C^.-.« !: » : ;»;;^::: : :: : : : , :::: ....\ S |H |N 



; | ] 

Here the first observer on the left stands upon Saturn, and the sun fills a compara- 
tively large angle, as shown at A. From Herschel the angle is smaller, and at Neptune 
it is still less. 

3. By applying the principles thus illustrated to the 3un, as viewed from the several 
planets, we are enabled to determine his comparative magnitude, as seen from each 
of these points. The following cut represents the comparative apparent magnitude of 

THE SUN AS SEEN FROM THE DIFFERENT PLANETS. 



From 
N. H. S. Jupiter. Mars. 



4. The relative apparent magnitude of the sun, as seen from different points in the 
Solar System, is as follows : 




From Jupiter 6' 

" Saturn 3|' 

« Herschel 1§' 

" Neptune 50" 



From Mercury 82£' 

" Venus 44|' 

" Earth 32' 

" Mars 21' 

" The Asteroids, say 12' 

5. Let us continue our imaginary journey outward, beyond Neptune, toward the 
fixed stars, and in a short time the glorious sun, so resplendent and dazzling to our 
view, will appear only as a sparkling star ; and the fixed stars will expand to view as we 
approach them, till they assume all the magnitude and splendor of the sun himself.] 



LESSOJST X. 

LIGHT AND HEAT OF THE PLANETS. 

17 9. From what source are the Light and Heat of the Planets 
derived ? 



PRIMARY ASTRONOMY. 



41 



From the Sun. 

180. What effect does the variation in their distances have upon 
them, in this respect ? 

It must make a great difference in their respective 

temperatures. 

[1. The following cut is designed to illustrate the 



PHILOSOPHY OF THE DIFFUSION OF LIGHT. 




Here the light is seen passing in straight lines, from the sun on the left toward the 
several planets on the right. It is also shown that A, B, and C receive equal quanti- 
ties of light, though B is four times and C nine times as large as A; and as the light 
falling upon A is spread over four times as much surface at B, and nine times as much 
at C, it follows that it is only one-ninth as intense at C, and one-fourth at B as it is at 
A. Hence the rale, that the light and heat of the planets is, inversely, as the squares 
of their respective distances. 

2. The student may not exactly understand this last statement. The square of any 
number is its product when multiplied by itself. Now suppose we call the dis- 
tances A, B, and C, 1, 2, and 3 miles. Then the square of 1 is 1 ; the square of 2 is 
4 ; and the square of 3 is 9. The light and heat, then, would be in inverse proportion 
at these three points, as 1, 4, and 9 ; that is, four times less at B than at A, and nine 

times less at C. These amounts we should state as 1, I, and 1.1 

'4' 9 J 

1§1. What is the comparative Light and Heat of the planet Mer- 
cury ? 

It must be about 6^ times as great as that of our 
globe. 

182. How high a Temperature would that be ? 

About 325 degrees, or 113° hotter than boiling water. 

[Taking the average temperature of our globe at 50 degrees, that of Mercury would 
be 6£ times 50, or 325. But water boils near the level of the sea at 212, and this 
being subtracted from 325, leaves 113 degrees.] 

183. What, then, must be the Light and Temperature of Neptune ? 
Only about ^ part as great as that of our globe. 

4*~ 



42 



PJBIMARY ASTRONOMY. 



[1. The comparative light and heat of the planets, the earth being 1, is as follows: 



Mercury 6£ 

Venus 2 

The Earth 1 

Mars £ 

The Asteroids £ 



Jupiter i 

Saturn 

Herschel 



3FS 



Neptune — * 



0(T0 



These last are degrees of coldness of which we can form no just conception. 

2. It is not certain, however, that the heat is proportionate to the light received by 
the respective planets, as various local causes may conspire to modify either extreme 
of the high or low temperatures. For instance, Mercury may have an atmosphere 
that arrests the light, and screens the body of the planet from the insupportable rays of 
the sun ; while the atmospheres of Saturn, Herschel, &c, may act as a refracting 
medium to gather the light for a great distance around them, and concentrate it upon 
their otherwise cold and dark bosoms.] 



LESSON XI 



MAGNITUDE, DENSITY, AJSTJ GRAVITATION OF THE PLANETS. 

1§4. What is meant by the Magnitude of a planet? 

Its size, bulk, or dimensions. 

1§5. What is the Diameter of a planet ? (See 72.) 

18<3. State the Diameter of the several planets so far as known. 

miles. 



Mercury 3,200 miles 

Venus 1,100 " 

The Earth 1,912 " 

Mars 4,200 " 

Flora 

Vesta 210 " 

Iris 

Metis 

Hebe 

Hygeia 



Astrsea 

Juno 1,400 

Parthenope 

Victoria 

Ceres 1,600 

Pallas 2,100 

Jupiter 89,000 

Saturn 79,000 

Herschel 35,000 

Neptune 35,000 



187. What is the bulk of Mercury as compared with our globe ? 
He is about J-g- as large. 

[The magnitudes of spherical bodies are to each other as the cubes of their diame- 
ters. Now 8,000 X 8,000 X 8,000 = 512,000, and 3,200 X 3,200 X 3,200 = 32,668. 
Then 512,000 -f- 32,668 = _J , nearly, the comparative bulk of Mercury.] 

188. What is the comparative size of Venus ? 



PKIMARY ASTEO]S T OMY. 43 

About t 9 q- that of the Earth. 

1§9. How do Jupiter, Saturn, Herschel, and Neptune compare 
with our globe? 

Jupiter is more than 1,400 times as large; Saturn 
1,000 times ; and Herschel and Neptune each 90 times. 

[This subject may be illustrated by the following cut, exhibiting the 

COMPARATIVE MAGNITUDE OF THE SUN AND PLANETS. 



J S /O H 

HI m © 



The student may think it almost incredible that what appears only as a star in the 
heavens should be larger than the mighty globe upon which he dwells ; but when he 
considers their immense distance, and remembers the effect it must have upon their 
apparent magnitude, as illustrated under Question 178, he will see that they could not 
be seen at all if they were not very large bodies.] 




DENSITY. 

190. What is meant by Density ? 
Compactness or closeness of parts. 

[1. For example, cork is less dense than iron, and stone is more dense than common 
earth. 

2. Density and solidity are not the same ; for while ice is more solid than lead, it is 
far less dense, and is consequently lighter.] 

191. What can you say of the Density of the planets ? 

They differ in the compactness of the substances of 
which they are composed. 

192. What is the Density of Mercury? 

About the same as lead, or three times the average 
density of our globe. 

193. State the Density of Venus and Mars. 

It is about the same as that of the Earth. 



44 PEIMAEY ASTBONOMY. 

194. How is it with the other large planets 1 

Jupiter and Herschel have but \ and Saturn T ^ the 
density of our globe ; the first two answering to water, 
and the latter to cork. 



GRAVITATION. 

195. What is Gravitation ? 

The tendency of all bodies toward each other. 

[Gravitation is a species of attraction, and is often called attraction, or the attraction 
of gravitation.] 

196. Give an example of the attraction of the earth. 
effect of Gravitation. 

It is seen when bodies 
raised from the earth, and 
left without support, fall 
to its surface. 




[All substances fall toward the earth's 
center from every part of the globe, as a 
spherical loadstone would attract parti- 
cles of steel to its surface in every di- 
rection. Hence when these four men, 
standing on different sides of the globe, 
drop each a stone, they all fall toward 
the same point, because the earth at- 
tracts them all to herself.] 

197. What constitutes the Weight of any substance? 
It is the force of attraction or gravitation. 

198. Upon what does the amount of this force depend 1 

Upon the quantity of matter* in the bodies attracting, 
and their distances from each other. 

199. Why does not a cubic foot of cork weigh as much as a 
cubic foot of lead ? 

Because, being much less dense, it contains much less 
matter to be attracted. 

* Matter, in philosophy, signifies any visible or tangible substance — 
as earth, wood, stone, water, (fee. 




PKttlARY ASTRONOMY. 45 

200. Suppose the Earth was only half as dense as she is, how 
would it affect, the weight of bodies at her surface 1 

It would be reduced one : half. 

201.- Suppose a stone, weighing four pounds at the Earth's sur- 
face, were taken down half way to her center, what would it weigh 
there ?* 

About two pounds. 

[In this cut the diameter of the earth is divided into four 
equal parts, C, D, E, and F. At A the whole attraction 
amounts to four pounds. When the stone reaches B, the 
part C attracts as strongly upward as D does downward, and 
their forces balance each other. Then as C and D mutually 
neutralize each other, we have only the parts E and F, or 
one-half the globe to attract the stone downward ; conse- 
quently the attractive force would be only half as great at B 
as at A, and the stone would weigh only two pounds.] 

202. What would it weigh at the Earth's center 1 
Nothing. 

203. Why? 

Because the attractive force would be the same in all 
directions. 

204. Would an object weigh the same on the surface of the 
several planets ? 

It would not. 

[It is intended merely to assert that the attractive force is not the same. If a body 
were actually weighed upon the surface of each planet, by scales, it would weigh the 
same on all ; because the force of attraction upon the weights would be just equal to 
that of the body to be weighed, whether it were more or less. With a steelyard it 
would be the same. A spring and hook, therefore, is the only instrument with which 
we could weigh objects accurately on all the planets.] 

205. Why not J 

Because they vary in hulk and density, and conse- 
quently in their attractive forces. 

[1. A body weighing one pound on the Earth would weigh 9£ oz. on Mercury, 
15 oz. on Venus, 8 oz. on Mars, 2 lbs. 8 oz. on Jupiter, 1 lb. 5£ oz. on Saturn, and about 
12£ oz. on Herschel. 

2. A person weighing 150 lbs. on the Earth, would consequently weigh 75 lbs. on 

* This question proceeds upon the supposition that a well be sunk 
2,000 miles deep without filling with water. 



46 PEIMAEY ASTRONOMY. 



Mars, 375 on Jupiter, &c. The attractive force of the Asteroids is so slight, that if a 
man of ordinary muscular strength were transported to one of them, he could probably 
lift a hogshead of lead from its surface without difficulty. 

3. In estimating the attractive force of a planet, we must consider both its balk and 
density. Though one planet were as large again as another, still if it were but half as 
dense, it would contain no more matter than the smaller one ; and their attractive 
force would be equal. If Jupiter, for instance, were as dense as the earth, his at- 
tractive force would be four times what it now is ; and if the density of all the solar 
bodies were precisely the same, their attractive force, or the weight of bodies on their 
surfaces, would be in exact proportion to their bulk.] 

206. How does Distance affect the attractive force of bodies ? 
The rule upon this subject is, that the attraction is in 

inverse proportion to the square of the distance. 

[If this should not be easily understood by the pupil, he may turn back to Question 
180 and the Note. Light and gravitation are governed by precisely the same law, so 
far as the effect of distance upon them is concerned ; so that the illustration of one, as 
given at page 41, will answer for both.] 

207. Suppose three bodies, whose distances from the Sun were as 
1 , 4, and 8, what would be their relative attraction 1 

lj T6"5 ailC ^ TZ- 



LESSON XII. 

REVOLUTIONS OF THE PLANETS ABOUND THE SUN. 
20§. Are the planets at rest or in motion ? 
They are all in motion, or revolution. 

209. Describe their motions. 

Each planet has two revolutions : one around the Sun, 
called its Periodical* revolution ; and another on its 
own axis, called its Div/mdl\ revolution. 

210. What is the time required for a planet to revolve around the 
Sun called ? 

Its Periodic Time. 



* Pe-ri-od'-ic-al, occurring at regular intervals, or at stated times 
hence weekly, monthly, or quarterly publications are called Periodicals. 
f Di-urn'-al, from the Latin diurnus, daily. 



PRIMARY ASTRONOMY. 47 

211. In what direction do the planets revolve around the Sun? 

Eastward, or toward that part of the heavens in 
which the Sun appears to rise. 

212. Can you state the Periodic Times of the several planets 1 

Mercury yrs. 88 ds. I Jupiter 11 yrs. 317 ds. 

Venus " 225 « j Saturn 29 " 175 " 

The Earth 1 or 365i '• j Herschel 84 " 

Mars 1 yr. 322 " j Neptune 164 " 

The Asteroids about 4-^- " 

[As the periodic time of a planet constitutes its year, it follows that Herschel's year 
equals 84 of ours, &c] 

213. What is the hourly motion of the planets in their orbits ? 
From 11,000 to 110,000 miles. 

214. Which have the most rapid motion? 

Those nearest the Sun. 

215. What is the hourly motion of the Earth ? 
About 68,000 miles. 

[1. It may seem incredible to the student that the ponderous globe is flying through 
space at the rate of 68,000 miles an hour, or some 30 times as swift as a bullet ; but, 
like many other astonishing facts in Astronomy, its truth can easily be demonstrated. 
The diameter of a circle is to its circumference as 7 is to 22 nearly. The Earth's dis- 
tance from the Sun being 95,000,000 miles (Question 175), it is obvious that the whole 
diameter of her orbit is twice that distance, or 190,000,000 ; then, as 7 : 22 : : 190,000,000 
to 597,142,857 miles, the circumference of the Earth's orbit. Divide this sum by 8,766, 
the number of hours in a year, and we have 68,108 miles as the hourly velocity of the 
Earth. 

2. As the Earth is not propelled by machinery like a steamboat, or borne upon 
wheels like a railroad car, it is not strange that we are insensible of its rapid motion, 
especially as every thing upon its surface, and the atmosphere by which it is sur- 
rounded, move onward with it in its rapid flight.] 

216. What keeps the planets revolving so steadily in their orbits J 
It is the result of two distinct influences, called the 

centripetal* and centrifugal\ forces. 

* Cek-trip'-e-tal, from centrum, center, and peto, to move toward : 
tending to the center. 

f Cen-trif'-c-gal, from centrum, and fugio : to fly from the center. 

fj^f In the above two cases we agree with Dr. Webster, that " the 
common accentuation is artificial and harsh. The accent on the first and 
third syllables, as in Circumpolar, would be natural and easy." 



48 



PELMAEY ASTRONOMY. 



2 17. What is the Centripetal Force? 

It is the mutual attraction of the Sun and planets, 
which prevents them from wandering off from the Sun. 

218. Describe the Centrifugal Force. 

It is the tendency to fly off from the Sun, produced 
by the planets revolving around him. 



CENTRIPETAL AND CENTRIFUGAL FORCES. 



CENTRIFUGAL 




[1. In the cut we have an arc of the 
Earth's orbit. The attraction of the sun, 
shown by the line A B, is the centripetal 
force, while the tendency to follow the line 
C D, is the centrifugal force. The two 
combining, give the planet what may be 
called a resultant motion, in the direction 
C E ; and as it meets no resistance in the 
void of space, it continues to revolve from 
age to age. 

2. If the centrifugal force were suspended, 
the planets would at once fall to the Sun ; 
and if the centripetal force were destroyed, 
the planets would fly off in straight lines, 
and leave the Solar System forever. 

Then might be realized the chaos and 
confusion of the poet : 

" Let Earth unbalanced from her orbit fly, 
Planets and suns run lawless through the sky, 
Let ruling angels from their spheres be hurled, 
Being on being wreck'd, and world on world."] 

219. Why do the planets nearest the Sun revolve most rapidly ? 
Because the nearer the Sun the greater the attraction 

(206), and the more centrifugal force is necessary to 
balance it. 

[The mechanism of the Solar System strikingly displays the wisdom of the great 
Creator. The centrifugal force depends, of course, upon the rapidity of the revolution ; 
and in order that these forces might be exactly balanced, God has imparted to each 
planet a velocity just sufficient to produce a centrifugal force equal to that of its gravi- 
tation. Thus they neither fall to the Sun on the one hand, nor fly off beyond the 
reach of his beams on the other, but remain balanced in their orbits between these 
two great forces, and steadily revolving from age to age.] 

220. What three great principles or laws are found to prevail 
throughout the Solar System ? 

The first is, that all the planets and comets revolve in 
elliptical orbits. 

221. State the Second law of planetary motion. 



PEIMARY ASTRONOMY. 



49 



RADIUS VECTOR. 
-©-'• 



The Radius Vector of a planet describes equal areas 
in equal times. 

222. What is meant by the Ra- 
dius Vector?* 

It is an imaginary line join- 
ing the center of the Sun and 
the center of the planet, in any 
part of its orbit. 

[In the cut, the lines extending from the Sun 




to the planet at different points, represent the 
radius vector in different positions.] 

223. What is meant by the Ra- 
dius Vector describing equal areas in 
equal times ? 

That it sweeps over the same surface in an hour, 
when a planet is near the Sun and moves swiftly, as 
when it is furthest from the Sun and moves most 
slowly. 

[In the preceding cut the twelve triangles, numbered 1, 2, 3, &c, over each of 
which the radius vector sweeps in equal times, are equal.] 

224. What is the Third great law? 

That the squares of the periodic times of any two 
planets are proportioned to the cubes of their mean dis- 
tances from the Sun. 

[1. The student will understand that the square of any number is the product of that 
number multiplied by itself; and the cube of a number is the result when that number 
is multiplied by itself, and the product multiplied by the same number again. 

2. By this law of proportion between the periodic times and the average distances 
of the planets, when one is ascertained the other can be deduced from it.] 

225. Who discovered these three great laws ? 

Kepler, a German astronomer, after whom they are 
called Kepler's Laws. 

\Kejtler was a disciple of Tycho Brake, a noted astronomer of Denmark, and was 
equally celebrated with his renowned tutor. His residence and observatory were at 
Wirtemburgh, in Germany.] 

* Vec'-tor, from veho, to carry : — a radius carried around. 



50 



PRIMARY ASTRONOMY. 



LESSON XIII 



ASPECTS, SIDEREAL AND SYNODIC REVOLUTIONS, ETC. 

226. What is meant by the Aspects of the planets ? 
Their positions with reference to each other. 

227. When are planets said to be in Conjunction ? 

When they are in the same longitude in the heavens. 



[1. If they are on the same 
meridian, or, in other words, 
are the same distance east or 
west, they are in the same 
longitude, and are said to be 
in conjunction. 

2. In this cut the Sun, 
Venus, and Mars are shown 
in conjunction; being, as 
viewed from the Earth, in the 
same longitude. The follow- 
ing is the sign of conjunction / 

6-] | d 

228. What is an «>a 



ASPECTS OF THE PLANETS. 

MARS IN CONJUNCTION 



6 '* 









WARS IN OPPOSITION 



Inferior Conjunction! \ 

It is when the \ 
planet is between 
the Earth and the 
Sun. (See Venus 
at " inferior" in the 
cut.) 

[This conjunction is called inferior, because the dark side of the planet is toward 
the Earth, and she shines with inferior brilliancy.] 

229. What is a Superior Conjunction ? 

It is when a planet is beyond the Sun, and its illu- 
minated side is toward us. (See Yenus at " superior.") 

230. When are planets in Quadrature? 
"When they are 90° apart. 

[Mars would be in quadrature with the Earth and Venus in the above figure, if placed 
at A. The sign for quadrature is □, as there represented.] 

231. WTien are planets in Opposition ? 



PEIMARY ASTRONOMY. 51 

When in opposite directions in the heavens, one 
toward and the other from the Snn. 

[In the lower part of the last cut Mars and Venus are in opposition. This aspect ia 
denoted by the sign g , as there shown.] 

232. What is meant by the Sidereal Revolution of a planet? 

It is a complete revolution from any given point in 
its orbit around to the same point again. 

233. Why is it called a Sidereal Revolution ? 

From dderalis,* because such complete revolution is 
determined by observations upon the fixed stars. 

234. What is a Synodic Revolution ? 

It is from one conjunction to the same conjunction 



[1. In the adjoining cut the revolution SIDEREAL AND SYNODIC revolutions. 

of the Earth from A, opposite the star 

B, around to the same point again, ,--•""" **»., 

would be a sidereal revolution. /' **v^ 

2. Suppose the Earth and Mercury to / ,.-''""" '**%, 

start together from the points A C / „•'*' ^ q, 

(where Mercury would be in inferior / /' /•** \-p **% 

conjunction with the Sun), and to pro- / / /' .o 

ceed in the direction of the arrows. In ; / / 



\3 



88 days Mercury would come around to f > ;' ^h 

the same point again ; but as the Earth i j ( I ^53? - -<jrl.. ^A ; j 

requires more than four times that num- \ \ \ \ / / ""V *'B 

ber of days for a revolution, she will \ \ \ *v -*' / / / 

only have reached the point D when V \ %v v. ,.-' / j 

Mereury arrives at C again ; so that they \ \ ""* '* 

will not be in conjunction, and a synodic \ **N» ^-*'' 

revolution will not be completed by \ -* 

Mercury. He starts on, however, in his ° < ***^^ ^^^ 

second round, and constantly gaining ' ' 

upon the Earth, till in 27 days from the time he left C the second time, he overtakes 

the Earth at E and F, and is again in inferior conjunction. 

3. From this illustration, it will be seen that the synodic revolution of a planet must 
always require more time than the sidereal. 

4. The term sxjnod signifies a meeting or convention ; and the synodical revolution of 
a planet is a meeting revolution ; that is, from one meeting or conjunction to another. 
By noting the synodic as a meeting revolution, its meaning will not be forgotten.] 

235. Which requires the most time, the Sidereal or the Synodic 
Revolution ? 



The synodic. 



* See Question 127 and Notes. 



52 PKIMAKY ASTRONOMY. 

236. What familiar illustration can you give? 

At twelve o'clock the hour and minute-hands of a 
clock or watch are together ; but at one o'clock, when 
the minute-hand has made a complete revolution, and 
points to XII. again, the hour-hand has gone forward 
to I., and the minute-hand will not overtake it till 
about five minutes afterward. 

237. Which revolution, then, represents the Sidereal and which the 
Synodic 1 

The revolution of the hour-hand from XII. to XII. 
again, is like the sidereal revolution of a planet; and 
when it overtakes the hour-hand, it becomes a synodic 
revolution. 

238. Stale both the Sidereal and Synodic periods of some of the 
planets. 

Sidereal. Synodic. 

Mercury 88 days 115 days. 

Venus 225 " 594 

Mars 1 year, 322 " 780 

Jupiter 11 " 317 " 399 

Saturn 29 " 175 " 378 

Herschel 84 " 369-£- 

Neptune 164 " 367£ 

239. Why is the Synodic period of the most distant planet the 
nearest to the periodic time of the Earth ? 

Because, being remote from the Sun, they move very 
slowly, and the Earth soon overtakes them after per- 
forming her periodic revolution. 

SYNODIC PERIODS OF THE EXTERIOR PLANETS. 

( $^:== i 



I 



PEIMAEY ASTRONOMY. 



53 



[Suppose the Earth and Herschel to he in conjunction, as shown at A B. In 365| 
days the Earth performs her sidereal or periodic revolution, and returns to the point 
A again. In the mean time Herschel, whose periodic time is 84 years, has passed 
through only ^ th part of his orbit, or about 4£° to the point C ; and in 4£ days the 
Earth overtakes him on the line D. It is on this account that the synodic period of 
Herschel is only 367* days; or 4£ days longer than the periodic time of the Earth.] 



LESSOR XIY 



DIRECT AND RETROGRADE MOTIONS, PLANETS STATIONARY 



ETC. 



DIRECT AND RETROGRADE MOTIONS. 




240. When is a plane? 's motion said to be Direct ? 
When it is eastward among the stars. 

[All the planets actually revolve eastward, as stated at 211, and they generally appear 
thus to revolve, though not always.] 

241. When is a planet said to Retrograde ? 

When it seems to go back or westward in the ecliptic. 

242. What is the cause of this apparent variation in the course of 
the planets ? 

It is due to the rapid rev- 
olution of the interior plan- 
ets around the Sun, and to 
the changes in the position 
from which we view them, 
produced by the Earth's 
revolution. 

[Suppose the Earth to be at A and 
Venus at B, she would appear to be at 

C, among the stars. If the Earth remained 
at A while Venus was passing from B to 

D, she would seem to retrograde from C 
to E ; but as the Earth passes from A to F 
while Venus goes from B to D, Venus will 

appear to be at G ; and the amount of her %, X 
apparent westward motion will only be "'v.j^— *> \ . 
from C to G.] """ -@ 

A 

243. What is meant by the Arc of Retrogradation , 

_ 




54 PEIMAEY ASTEONOMY. 

It is the portion of the ecliptic through which a 
planet seems to retrograde. 

[In the preceding figure it would be the arc C G.] 

244. When is a planet said to be Stationary ? 

"When it appears to move neither east nor west among 
the stars. 

[1. For a short time, when Venus is at B, she will be coming toward the Earth, and 
at D she will be going from the Earth ; so that she will appear to remain stationary at 
C and E. 

2. Some late writers have called this a stationary motion ; for instance, one asks, 
" When is a planet's motion said to be stationary ?" We were not before aware that 
no motion at all was a stationary motion. See Clark's Astronomy, p. 15, and Smith's 
Illustrated, p. 12.] 

245. What is meant by the greatest Eastern and Western Elonga- 
tions of a planet ? 

It is the greatest apparent distance east or west of the 
Sun at which it is ever found. 

[In the last cut the point B would represent the greatest eastern and D the greatest 
western elongation of Venus.] 

246. What is the greatest angular distance to which Venus ever 
departs from the Sun? 

She varies from 45 to 48 degrees. 

247. What does this variation in her elongations indicate? 

That she revolves in an elliptical orbit. 

[1. It will be obvious, without illustration, that if she is further from the Sun at 
one time than at another (as is evident from the difference in her elongations), she can- 
not revolve in a circle, and her orbit must be elliptical. 

2. The eccentricity of her orbit is ascertained by observing the difference between 
her greatest and least distance, which is only about 3°. Her orbit, therefore^ is very 
nearly a circle.] 

248. When is Venus Morning Star ? 

When she is west of the Sun, and rises before him. 

[She must be west of the Sun, of course, from her inferior to her superior conjunc- 
tion. See cut, page 50.] 

249. When is she Evening Star? 

"When she is east of the Sun, and remains above the 
horizon after he has gone down. 

[From her superior to her inferior conjunction she is east of the Sun, and Morning 

Star.] 



PRIMARY ASTRONOMY. 



55 



VENDS AS MORNING AND EVENING STAR. 
INI 



c.-' 



■■■'\\l/j- 



J- : tfvST..-#^i-.^sr ; F 




[Let the student hold the book up south of him, and he will at once see why Venus 
is alternately Morning and Evening Star. Let the plane A B represent the sensible or 
visible horizon, C D the apparent daily path of the Sun through the heavens, and E 
the Earth in her apparent position. The Sun is shown at three different points; 
namely, rising in the east; on the meridian; and setting in the west: while Venus is 
seen revolving around him from west to east, or in the direction of the arrows. Now 
it is obvious that when Venus is at F, or west of the Sun, she sets before him as at G, 
and rises before him as at H- She must, therefore, be Morning- Star. On the other 
hand, when she is east of the Sun, as at J, she lingers in the west after the Sun has 
gone down, as at K, and is consequently Evening- Star. 

2. In this cut, Venus would be at her greatest elongation eastward at J, and west- 
ward at F ; and in both cases would be " stationary." At L and M she would be in 
conjunction with the Sun. 

3. Were the Earth to suspend her daily rotation, with the Sim on the meridian of 
the observer, as represented at L, we might readily watch Venus through her whole 
circuit around the Sun. 

4. Venus may sometimes be seen at mid-day, either east or west of the Sun ; and 
Dr. Dick considers the day-time most favorable for observing her with a tele- 
scope.] 

250. How long is Venus alternately Morning and Evening Star ? 
For 292 days, or from one conjunction to another. 

251. What did the ancients think of the Morning and Evening 
Stars? 

They supposed they were two different stars. 

252. What did they call them 1 

They called the Morning Star Phosphor, and the 
Evening Star Hesperus. 



56 



PEIMAEY ASTRONOMY. 



NATURAL APPEARANCE OF VENUS IN DIFFERENT POSITIONS. 




[1. On the left Venus is seen in the east as Morning Star, at various distances from 
the Sun, and on the right as Evening Star. 

2. We would earnestly recommend to the student to ascertain where Venus is at the 
time he is learning this lesson, and to watch her for a few weeks, and see if her move- 
ments do not answer to the description here given.] 

253. What is the greatest elongation of Mercury ? 
It varies from 16 to 29 degrees. 

[This proves the orbit of Mercury also to be elliptical.] 

254. Is Mercury often seen ? 
He is not. 

255. Why not 1 

Because generally so near the Sun as to be hid from 
our view by bis beams. 

256. In what months must Mercury he seen, if at all? 
In March and April, August and September. 

[By consulting an Almanac, you can ascertain when he is at his greatest elongation, 
and if it is eastward, look out for him low down in the west, just after sunset. If his 
elongation is westward, he must be looked for in the east, before sunrise. It will be 
worth rising early to see him.] 

257. How do you account for the apparent retrograde motion of 
the Exterior planets ? 

It is caused wholly by the change of the Earth's 
position, in revolving around the Sun. 



PRIMARY ASTRONOMY. 57 



RETROGRADE MOTIOX OF THE EXTERIOR PLANETS. 



A 
--.. 



if© t 



F 



t 



f_l. Suppose the Earth at A, and the planet Neptune at B, he would then appear to 
be at C, among the stars ; but as Neptune moves but a little from B toward F, while 
the Earth is passing from A to D, Neptune will appear to retrograde from C to E. 
Whatever Neptune may have moved, however, from B toward F, will go to reduce the 
amount of retrogression. 

2. Tt is obvious from this figure, that the more distant an exterior planet is, and the 
slower it moves, the less will be its arc of retrogradation, and the longer will it be 
retrograding. Neptune appears to retrograde 180 days, or nearly half the year. 

3. The student will now see the philosophy of the following table, in which may be 
seen the amount of arc and the time of retrogradation of the principal planets : 

Arc. Days. 

Mercury 13^° 23 

Venus 16 42 

Mars 16 73 

Jupiter., 10 121 

Saturn 6 139 

Herschel 4 151 

Neptune 1 180] 



LESSON XY 



25§. What is meant by tlie Diurnal Revolution of a planet 1 

Its revolution upon its own axis, causing day and 
night. 

[The regularity with which the Earth revolves upon her axis, is referred to in the 
following beautiful language of the prophet: "Thus saith the Lord, If ye can break 
my covenant of the day, and my covenant of the night, and that there should not be 
day and night in their seasons ; then may also my covenant be broken with David," 
&c. Jeremiah xxxiii. 20.] 



259. On which side of a planet is it Day ? 
On the side toward the Sun. 

260. Where is it Night 1 

On the side opposite the Sun. 



58 PEIMART ASTRONOMY. 



PHILOSOPHY OF DAT AND NIGHT. 




261. What, then, does the time of a planet's revolution upon its 
axis constitute ? 

Its day / including a day and a night. 

262. What is meant by a Solar Day 1 

It is the time elapsing from the Sun's crossing a me- 
ridian, to his coming to the same meridian again. 

263. How long does this require ? 
Twenty-four hours. 

264. What is a Sidereal Day 1 

It is the time required for the apparent revolution of 
a star from the meridian around to the same meridian 
again. 

265. What is the length of a Sidereal Day ? 
Twenty-three hours, 56 minutes, and 4 seconds. 

266. What, then, is the difference between a Solar and Sidereal 
Day? 

About four minutes, the solar day being the longest. 

267. Will some one of the class explain the cause of this by a 
diagram upon the blackboard ? 

SOLAU AND STDEREAL TIME. 
_ SIDEREAL DAY, 23 h. 56 m. 4 S._ 

S L AK^Y,24b, - 

v$% 

->g SUN ON THE MERIDIAN. 




[1. To the man at A the Sun (S) is exactly on the meridian, or it is twelve o'clock, 
noon. The Earth passes on from B to D, and at the same time revolves on her axis. 



PRIMARY ASTRONOMY. 59 

When she reaches D, the man, who has stood on the same meridian, has made a 
complete revolution, as determined by the star G (which was also on his meridian at 
twelve o'clock the day before), but the Sun is now east of the meridian ; and he must 
wait four minutes for the Earth to roll a little further eastward, and bring the Sun again 
over his north and south line. 

2. It is obvious that if the Earth was not revolving around the Sun, her solar and 
sidereal days would be the same ; but as it is, she has to perform a little more than one 
complete revolution each solar day, to bring the Sun on the meridian.] 

268. What is the annual difference between Solar and Sidereal 
Time ? 

It amounts to one day in every 365J. 

269. Why is this? 

Because it takes 366 actual revolutions of the Earth, 
as measured by the fixed stars, to produce 365-j natural 
days. 

270. How does Longitude on the Earth affect our local timet 
Every 15° east makes it an hour earlier, and every 

15° west an hour later. 

271. Why is this I 

Because, if the Sun pass through 360° every 24 
hours, he must pass over 15° each hour; as 360° "-f- 
24 = 15°. 

272. When it is sunrise at New York what time is it 90° east of 
New York? 

Twelve o'clock. 

273. When it is 12 o'clock at New York what time is it 30° west 

of that point ? 

Ten o'clock. 

274. In what time does each planet revolve upon its axis ; or, in 
other words, what is the length of a day upon each of the planets ? 



Mercury....... 24 hour3. 

Venus 23£ " 

The Earth 24 « 

Mars. 24£ " 



The Asteroids unknown. 

Jupiter 10 hours. 

Saturn 10£ " 

Herschel and Neptune, unknown. 



275. How was it ascertained that the planets revolve on their 
respective axes ? 



60 



PBIMARY ASTRONOMY. 



DIURNAL REVOLUTION OF 
THE PLANETS. 



By observing the motion of spots upon their surfaces, 
by the aid of the telescope. 

276. In what direction do the planets ro- 
tate* on their respective axes? 

JEasfovard ; or in the same direc- 
tion that they revolve in their orbits. 

[1. In the cut we have an arc of the Earth's orbit, 
and the Earth revolving on her axis as she revolves 
around the Sun. The arrows show the direction in 
both cases. 

2. By holding the book up south of him, and looking 
attentively at the cut, the student will understand why 
the Sun " rises" or first appears in the east. It is be- 
cause the Earth revolves eastward. Thus the observer 
at A is carried round into the light, and sees the Sun 
rise when he reaches B.] 

277. Upon what four planets are tlie days nearly equal ? 
Mercury, Yenus, the Earth, and Mars. 

27 §. If Jupiter's day is only ten hours long, and his year equal 
to about twelve of our years (11 years y 317 days), how many days 
must he have in one of his years ? 

About 10,397. 

[In this computation we reckon 365 days to a year, and 24 hours for a day. Tf the 
student thinks we have too many days for a year, in any part of the Solar System, he 
will please reckon it for himself, and see if we are correct.] 

279. How many days has Saturn in one of his years 1 

About 25,000. 

[29 years 175 days == 10,760 days of our time; X 24 = 258,240 hours -f 10^ hours, 




the time of Saturn's revolution, 



24,594_3_ 



the number of days in his year.] 



2§0. What effect has the Rotary] motion of the planets upon 
their form or figure 1 

The centrifugal force produced thereby causes them 
to swell out at their respective equators, and to contract 
at their poles; thus giving them the form of oblate 
spheroids. 



* Ro'-tate, from rota, to revolve or move round a center. 
f Ro'-ta-ry, from rota, a wheel. 



PRIMARY ASTRONOMY. 



61 




[1. When fluids are left free to yield to the 1N revolution. 

influence of attraction, as mutually existing be- 
tween their particles, they invariably assume a 
spherical form. Hence water, in falling from 
the clouds, takes the form of spherical drops ; 
and melted lead, thrown from the top of a shot- 
tower, takes a spherical form, and cooling in 
the air on its passage down, remains perfect 
little globes, called shot. 

2. Take a ball of India-rubber, pass a rod 
through its center, and attach it to machinery, 
so as to give it a rapid rotary motion. When 
at rest it will be a sphere ; but when in motion it will contract at its poles, and swell 
out at its equator, thus becoming an oblate spheroid; and its oblateness will be in 
exact proportion to the rapidity of its revolution. 

3. A solid sphere would never become oblate by revolution. It might burst, from 
its powerful centrifugal tendency, as grindstones sometimes do in manufactories of 
cutlery; but it must be fluid, or at least soft and yielding, in order to become oblate 
by revolution. 

4. The oblateness of the planets, then, seems to indicate two things: First, that 
they were all once in a fluid or plastic state ; and, secondly, that they began to revolve 
while in that state, or before any part of them had become solid, like our continents 
and islands. 

5. So far as the Earth is concerned, we are taught in the Holy Scriptures — the best 
and most accurate of all books — that the earth and water of our globe were once so 
mixed, that the whole appeared as a " void" of " waters ;" and that they were after- 
ward separated into "earth" and "seas," by the Almighty Creator. (See Genesis i. 2, 
9, 10.) Thus we see that true science and the Bible are always in harmony with each 
other.] 

2§1. What is the difference, so far as known, between the Equa- 
torial and Polar Diameters of the several planets ? 

The Earth 26 miles, Mars 25, Jupiter 6,000, and 
Saturn 7,500. 

[The oblateness of Jupiter and Saturn is as plainly visible through a telescope, as 
the difference in the following figures is to the eye of the student. 



ORIGINAL FORM. 



PRESENT APPEARANCE. 




The plain line in the middle figure shows the original form, and the dotted line its 
present form. The difference is the change produced by its rotation. When measured 



62 



PELMAEY ASTKONOMY. 



of his average diameter ; and that being 89,000 miles, _} is but little less than 6,000.] 

2 §2. To what is the great oblateness of Jupiter and Saturn at- 
tributable 1 



CENTRIFUGAL FORCE. 




To their rapid revolutions upon 
their respective axes. 

[The tendency of a rotary motion to engender cen- 
trifugal force is illustrated in the adjoining cut, where a 
boy is seen turning a grindstone so rapidly, as to throw 
the water from its surface in every direction. In the 
same manner an increased motion on its axis would 
make a planet more oblate ; and an increased velocity 
around the Sun would cause them to leave their orbits, 
and fly off in a tangent, as stated in note after 218.] 

2 §3. If the Earth is oblate, what portions of its surface are nearest 
to its center ? 

Those about the Poles. 

2 §4. Does not the surface ascend then, on the whole, from the Poles 
to ilie Equator ? 

It does. 

2 §5. How, then, can rivers run from the Poles toward the Equa- 
tor, for any great distance, without running up hill? 
They cannot. 

[This ascent would be imperceptible in short distances, and where the bed of a 
river running south from a northern source actually inclined downward ; and yet there 
are circumstances, as we shall see, under which the above answer is strictly correct.] 

286. Can you give an instance of a river running up hill ? 

The Mississippi is said to be -higher at its mouth than 

it is some thousands of miles above. 

[The plausibility of this opinion may be illustrated 
by a diagram. Let A B represent the polar, and 
CD the equatorial diameters. The entire difference 
between them is 26 miles, or 13 miles on each side. 
The two circles represent this difference. Now as 
the Earth's circumference is 25,000 miles, the dis- 
tance from the poles to the equator (being one- A 
fourth of that distance) must be 6,250 miles ; and in 
that 6,250 miles the ascent is 13 miles, or over two 
miles to every 1,000 toward the equator. The Mis- 
sissippi runs from the 50th to the 30th degrees of 
north latitude inclusive, or 21 degrees; which, at 
69£ miles to a degree, would amount to about 1,500 




PRLMAEY ASTRONOMY. 63 

miles. If, then, it runs a distance equivalent to 1,500 miles directly south (in a wind- 
ing course of about 3,000), theory requires that it should be about three miles higher at 
its mouth, than it is 1,500 miles directly north. There is some philosophy, therefore, 
in saying that if a river runs for a great distance from either pole toward the equator, 
it must run up hill.] 

287. What causes the waters to flow toward the Equator, if they 
have to ascend in so doing? 

The centrifugal force imparted to them by the Earth's 
revolution. (See 280 and Illustrations.) 

288. What, then, would be the result if the Earth should cease 
to revolve on its axis? 

The waters of the equatorial regions would rush 
toward the Poles, till the Earth again became a perfect 
sphere. 

289. What would be the effect if the rotation of the Earth upon her 
axis was greatly increased ? 

The waters of the globe would rush toward the equa- 
tor, and the weight of bodies there would be greatly 
diminished. 

[1. The force of gravity and the centrifugal force are mutual opposing powers, act- 
ing against each other. The present rotation of the Earth diminishes the weight of 
bodies at the equator -o^th part, so that if the Earth had no such motion, bodies at 
her equator would weigh one pound in every 289, mare than they now do. 

2. Should the Earth revolve on her axis every 84 minutes, the centrifugal force 
would balance that of gravitation, so that bodies at her equator would be without 
weight ; and if the centrifugal force was still further increased by a still more rapid 
revolution, gravitation would be completely overpowered, and all fluids and loose sub- 
stances near the equator would fly off from the surface, as the water flies from the 
grindstone when the boy turns too fast on the opposite page.] 



LESSON XVI. 

THE ECLIPTIC, ZODIAC, SIGNS, LONGITDDE, ETC. 
290. What is the Ecliptic ? 

It is the plane of the Earth's orbit, or of the path in 
which the Sun appears to move in the heavens. 

[1. If the student does not fully understand what is meant by " the plane of the 
Earth's orbit," let him turn back and review Questions 32 to 37, and notes. 



64 



PEIMAEY ASTE01ST0MY. 



2, It is obvious that the centers of all circles or ellipses must be in the planes of 
such circles, and as the Earth revolves around the Sun, he, being in the center, must 
be in the plane of the Earth's orbit ; so that the ecliptic and the apparent path of the 
Sun must coincide.] 

291. Why is the plane of the Earth's orbit called the Ecliptic? 
Because eclipses of the Sun and Moon never take 

place except when the Moon is in or near this 
plane. 

292. What is the position of the Ecliptic to persons north of the 
Equator ? 

It is south of us ; runs east and west ; cuts the center of 
the Sun and Earth ; and may be imagined as indefinitely 
extended. (See Note 1 to Question 32.) 

[1. The precise position of this plane may always be 
known by the position of the Sun, which varies its distance PLANE 0F THE ECLIPTI c- 
north or south at different seasons of the year. The cause of 
this variation will be explained hereafter. 

2. In the adjoining cut an attempt is made to represent 
the ecliptic, or plane of the Earth's orbit. It is an oblique 
view, which makes the orbit appear elliptical. It shows 
one-half of the Sun and half the Earth on one side, and half 
on the other, as above stated. The circle, projecting beyond 
the orbit, is to represent the plane or ecliptic indefinitely 
extended.] 

293. What is meant by Above and Below the 
Ecliptic ? 

The Northern is called the upper 
and the Southern the lower sides. 

[The student must bear in mind, however, that there is no 
absolute up or down in the universe. (See 112 and Notes.) 
He must also guard against the idea that the ecliptic may be 
horizontal. This term has reference only to the Earth, and 
is descriptive of a plane depending altogether for its own 
position upon that of the observer, as shown and illustrated 
at 25. Though the ecliptic is a permanent plane, and cuts 
the starry heavens around us at the same points from age to 
age, it has no absolute up or down, unless it should be the 
direction to and from the Sun. The distinction of above and 

below is merely arbitrary, and grows out of our position north of the equator, which 
makes the south side of the ecliptic appear down to us.] 

294. What is the Zodiac? 

It is an imaginary belt, 16° wide— namely, 8° on 




PRIMARY ASTRONOMY. 



65 



each side of the ecliptic — and extending east and west 
quite around the heavens. 

OBLIQUE HORIZONTAL VIEW OF THE ECLIPTIC AND ZODIAC. 





..-0 ^;rr;;; ci --0--.. r 



[In this cut the interior dotted circle represents the Earth's orbit; the exterior the 
plane of her orbit extended to the starry heavens. The dark lines each side of the 
ecliptic are the limits of the zodiac. The Earth is shown in perspective, largest near 
to us, and growing smaller as her distance is increased. The arrows show her di- 
rection.] 

295. What is meant by the signs of the Zodiac ? 

They are mere divisions of the circle, each of which 

constitutes one-twelfth part. 

[1. The student will consult the definition of a sign and the illustration at Question 



cut by the perpendicular 



75. 

2. The twelve signs of the zodiac are divided off in 
lines.] 

296. How are the different signs of the Zodiac designated ? 

By specific names given to each, and by correspond- 
ing symbols or signs. 

29?. What are the names of the twelve signs, and their astro- 
nomical symbols? 



Aries (or the Ram) °C 

Taurus (the Bull) 8 

Gemini (the Twins) TT 

Cancer (the Crab) 3d 

Leo (the Lion) SI 

Virgo (the Virgin) TTg 



Libra (the Balance) =£h 

Scorpio (the Scorpion) 171 

Sagittarius* (the Archer) $ 

Capricornus (the Goat) V3 

Aquarius (the Waterman) .... %% 
Piscesf (the Fishes) }£ 



[I. These names being from the Latin, their signification is added in brackets, and 
should be understood by the pupil. In reciting, however, it is only necessary to give 
the first names— as Aries, Taurus, Gemini, &c. 

2. By carefully observing these symbols, the student will detect a resemblance be- 



* Sag-it-ta'-ri-us, from sagitta, an arrow. 



f Pi'-sces. 



6* 



66 



PRIMARY ASTRONOMY. 



tween several of them and the objects they represent. For instance, the sign for 
Aries represents his horns ; so also with Tamus, &c] 

20§. Why were these names given to the different signs ? 

Because the ancients imagined that the clusters of 
stars in each sign resembled the several objects after 
which they are named. 

[On this account they gave the name zodiac to this belt around the heavens. Not, 
as some have imagined, because it was a zone, but from the Greek zoun, an animal, 
because so many animals were represented within its limits.] 

299. In what order are these signs arranged? 

Beginning at Aries, they proceed eastward around to 

Pisces. 

PERPENDICULAR VIEW OF THE ECLIPTIC. 

\0 9 350 

■% 




C ' 9/ o'a o'ei o"6\ ^ 

[1. On pages 64 and 65, we presented oblique views of the ecliptic. The above is 
a perpendicular view. The Sun is seen in the center, and the Earth revolving around 
him ; and in the distance is shown the circle of the starry heavens. 

2. This circle is divided into twelve equal parts, representing the twelve signs of the 
zodiac. 

3. The object, which the stars in each sign were supposed to resemble, is placed in 
that sign, and the symbol immediately opposite and within the sign.] 

300. What influence have these signs upon health, vegetation, or 

any other terrestrial objects? 

None at all. 



PRIMARY ASTRONOMY. 



6*7 



ANCIENT ASTROLOGY. 



[1. The ancienls believed in a pretended science 
called Astrology, and taught that the stars exerted a 
controlling influence over the destinies of mortals. 
A fragment of this barbarous superstition may still 
be met with occasionally in the pages of an al- 
manac, designed to show which part of the human 
body each sign " governs." The annexed cut is a 
representation of this heathen absurdity. What an 
idea for any civilized nation to indulge, that a clus- 
ter of stars, millions of miles distant, govern the 
arms or feet of men ! 

2. This picture has been published in almanacs, 
till many people actually think there is some truth 
in astrology. Hence we sometimes hear them talk 
of doing things " when the sign is right," or when 
it is " in the head," or " in the heart." This, also, is 
founded in error and superstition. The Sun is in certain signs at the same time every 
year, so that the place of the sign indicates a certain time, as much as any given day of 
the month ; and as certain things should be done at certain times, in order to succeed 
well, it is erroneously concluded that it is because "the sign is right." 

3. Impostors often take advantage of this credulity, and profess to "tell fortunes," as 
they call it, by the aspects of the planets, signs, &c. All these things are based upon 
erroneous notions respecting the influence of the stars upon our globe and its in- 
habitants, and should be rejected.] 




301. What is Celestial Longitude ? 

Distance east of a given point in the heavens. 

302. How is it reckoned 1 

From the first degree of Aries, around to the same 
point again, or to 360 degrees. 

[I. Suppose Aries to be on the meridian, as represented celestial meridians and 
page 66. Let the pupil hold his book up to the south longitude. 

of him, and the surface of the page will represent the 
plane of the ecliptic ; and the reckoning of 10, 20, 30, 
&c, from the top of the cut eastward, will answer to 
the manner in which celestial longitude is reckoned 
eastward around the heavens. 

2. The subject may be still further illustrated by ref- 
erence to the adjoining cut, in which the celestial con- 
cave is represented as a hollow sphere, with its meridians ; 
and the equinoctial extending to them in every di- 
rection. Let the meridian at the top represent the first 
degree of Aries. Begin at that point, and reckon 
toward you, and 90° will bring you opposite the axis of 

the Earth, and 90° more, or 180° in all, to the bottom of the figure. You are then 
half way around the zodiac, and 180° more, apparently from the bottom upward, 
on the other side of the cut, will bring you to 360°, or the point from which you 
started. 

3. If the observer stood on the upper side of the Earth in the figure, the 90th degree 
of longitude would be east, the 180th under his feet, in the heavens beyond the earth ; 
the 270th west, &c] 




68 



PRIMARY ASTRONOMY. 



30 3. Upon what does the apparent Longitude of a planet depend? 
Upon its position in the ecliptic and the point from 
which it is viewed. 

304. What is Geocentric* Longitude ? 

It is the apparent longitude of a planet when viewed 
from the Earth. 

305. What is Heliocentric] Longitude ? 

It is the longitude of an object as seen from the Sun. 

GEOCENTRIC AND HELIOCENTRIC LONGITUDE. 




[In this cut, the planet B, when viewed from the Earth at A, seems to be in the sign 
o^ ; but when viewed from the Sun, it appears to be in JJ. Again : when at C, her 
apparent longitude from the Earth is in fr| ; when from the Sun, she appears to be in 
/ . The reader will not only perceive the difference between geocentric and helio- 
centric longitude, but will see why the latter more than the former indicates the true 
position of the planet. It is an easy thing, however, if one is known, to deduce the 
other from it.] 



* Ge-o-cen'-tric, from the Greek ge, the Earth, and kentron, center : 
from the Earth, as the center or point of observation. 

f He-li-o-cen'-tric, from the Greek helios, the Sun, and kentron, cen- 
ter. 



PEIMAEY ASTEONOMY. 69 

LESSON XVII. 

FORM AND POSITION OF THE PLANETARY ORBITS, NODES, ETC. 

306. Are the orbits of the planets perfect circles ? 

They are not, but are all more or less AP * K "° N " 
elliptical. 

307. What is the point nearest the Sun ! 
called ? I 

The Perihelion* \ (P^k 

308. What is the most remote point called ? 

The Aphelion^. *--...--•*' 

- t PERIHELION. 

309. What is meant by the mean distance of a planet? 

It is the average between its greatest and least dis- 
tances. 

[The distances given on page 39 are the mean distances.] 

310. Do the planets revolve with a uniform velocity throughout 
their respective orbits? 

They do not. 

311. In what part do they move most rapidly? 
When nearest the Sun. 

312. And where most slowly ? 
When most distant from the Sun. 

313. Why is this? 

Because from the Aphelion to the Perihelion points 
the centripetal force combines with the centrifugal to 
accelerate^: the planet's motion ; while from Perihelion 
to Aphelion points, the centripetal acts against the cen- 
trifugal force, and retards^ it. 

* Per-i-hel'-ion, from peri, about, and helio, the Sun 

\ A-phel'-ion, apo, from, and helio, the Sun. 

% To hasten or cause to move fast. 

§ To delay, hinder, or render more slow. 



I 



70 PEIMAEY ASTEONOMY. 



[1. From A to B in the diagram, the centrifugal , 

force, represented by the line C, acts with the tend- ..-"* A 

ency to revolve, and the planet's motion is accele- 
rated ; but from B to A, the same force, shown by 
the line D, acts against the tendency to advance, 
and the planet is retarded. Hence it comes to Aphe- j 
lion with its least velocity ; and to Perihelion with ; \ 
its greatest. : *«V 

2. In the statement of velocities on page 47, the ; | 
mean or average velocity is given.] 'i \ *\ ( .■ 

314. Are the orbits of all the planets \ fp2|J 
in the same plane ? \ / 

They are not. \ x B y 

315. As they all revolve around the 

Sun as a common center, what is the consequence of their not revolving 
in the same plane ? 

They cut or pass through the plane of the Earth's 
orbit. 

VENUS PASSING THE PLANE OF THE EARTH'S ORBIT. 
L 



'N 

316. What are the points called where a planet passes the 
ecliptic ? 

The Nodes of its orbit. 

317. How are the Nodes situated? 

In opposite sides of the ecliptic, or 180° apart. (See 
preceding cut.) 

318. What is meant by the line of the Nodes? 

It is an imaginary line passing from one node to the 
other throngh the Sun's center. (See the line L N in 
the last cut.) 

319. How are the Nodes distinguished? 



PRIMARY ASTRONOMY. 11 

Into ascending and descending. 

320. Describe each. 

The ascending is the one through which the planet 
passes in coming above or north of the ecliptic ; and the 
descending, that through which it passes in returning 
south of the ecliptic. 

321. What characters are used to denote each? 

The ascending is indicated by &, and the descending 
by y. (See last cut.) 

[These characters should be drawn upon a blackboard by the Teacher, or some one 
of the class.] 

322. Are the Nodes of all the planetary orbits in the same Longi- 
tude? 

They are not ; but are distributed around the eclip- 
tic. 

323. How do we describe the position of the several planetary 
orbits ? 

By taking the ecliptic as the standard, and recording 
their deviation from it. 

324. How is this deviation ascertained? 

By marking the greatest distance from the ecliptic at 
which the planet is ever seen. 

325. What is the deviation of the several orbits from the plane 
of the ecliptic ? 



Mercury 7° 

Venus 3£ 

Mars 2 

Flora 6 

Vesta 7 

Iris 5 

Metis 6 

Hebe 15 

Astraea 5 

Juno 13 



Ceres 11° 

Pallas 344 

Hygeia , . 4 

Parthenope 

Clio 8i 

Jupiter 1\ 

Saturn 2-f- 

Herschel § 

Neptune 1| 



72 



PEIMAEY ASTEONOMY. 



INCLINATION OF THE ORBITS OF THE SEVERAL PLANETS TO THE PLANE OF 
THE ECLIPTIC. 




[1. In this cut the large line in the center represents the plane of the ecliptic, in 
which the Earth is seen on the right and left. 

2. The dotted lines crossing the ecliptic at the Sun's center, represent the plane of the 
orbits of several of the planets, and their inclination to the ecliptic. There are so 
many of them, and the inclination of several is so nearly alike, that it is impossible to 
represent them all in the same figure. The orbits of Mars, Jupiter, Saturn, Herschel, 
and Neptune are so near the ecliptic, that it would be difficult to represent their 
positions at all, except upon a very large scale. 



PERSPECTIVE VIEW OF THE PLANETARY ORBITS. 



JONO 13 




3. A drawing similar to the above may be found in Long's Astronomy, vol. i. p. 203 ; 
in Smith's Quarto, p. 40, and in several other modern compilations. It may help to 
form an idea of the inclination of the planetary orbits ; but we must guard against 
the impression it may make that all the planetary nodes are in the same part of the 
ecliptic, as we were obliged to represent in the cut. Instead of this, they are dis- 
tributed all about the ecliptic. Again : the cut shows the several planets at about 
the same distance from the Sun, contrary to the fact, as stated after Question 175 ; 
but, with these exceptions, it is a good illustration.] 



PBIMARY ASTEONOMY. ^3 



LESSOR XVIII. 
OF TRANSITS. 

326. What is a Transit ? 

The passage of a heavenly body over the meridian of 
any place, or across the disk- of the Sun. 

[This term is sometimes used with reference to terrestrial objects, as when we speak 
of the transit or passage of goods through a country. The words transition, transi- 
tive, transitory, &c, are derived from the primitive word transit.] 

327. When do planets appear to pass over the Surfs face ? 
When they pass directly between ns and him. 
32§. Do all the planets make transits across the Sun's disk ? 
They do not.f 

329. Why not? 

Because the exterior planets can never get between 
the Earth and the Sun. 

[Let the student turn back to 137, and to the cut, page 9.] 

330. Whit planets, then, make such transits ? 
Only Mercury and Venus. 

331. Do these make a transit at every revolution? 
They do not, 

332. Why not ? 

Because the planes of then* respective orbits do not 
lie in the plane of the ecliptic. 

[The student will see at once that if the planets all revolved in the same plane, like 
rolling so many bullets around an apple upon the top of a table, Mercury and Venus 
would seem to pass over the Sun's face at every revolution. But as one half of each 
of their orbits is above and the other half below the ecliptic, they will generally appear 
to pass either above or below the Sun. To illustrate : 



* Disk, the face or visible projection of a heavenly body, 
f That is, to our view ; though they may to the inhabitants of the ex- 
terior planets. 



74 



PRIMARY ASTRONOMY. 



«D 



,#©41 



— »E 



Let the right line A, joining the Earth and the Sun in the above diagram, represent 
the plane of the ecliptic. Now when an interior planet is in this plane, as shown at A, 
it may appear to be upon the Sun's disk ; but if it is either above or below the ecliptic, 
as shown at B and C, it will appear to pass either above or below the Sun, as shown 
at D and E.] 

333. Under what circumstances, then, do transits occur ? 

When the Earth and an interior planet meet on the 
same side of the ecliptic ; the planet being at its node, 
and the Earth on the line of the nodes. 

PHILOSOPHY OF TRANSITS. 
L 




[This cut represents the ecliptic and zodiac, with the orbit of an interior planet, his 
nodes, &c. The line of his nodes is, as shown, in the 16° of g and the 16° of -fri . 
Now if the Earth is in g f on the line L N, as shown in the cut, when Mercury is at 
his ascending node (£^) he will seem to pass upward over the Sun's face, like a dark 
spot, as represented in the figure. On the other hand, if Mercury is at his <ff when 
the Earth is in the 16° of fTl , the former will seem to pass downward across the disk of 
the Sun.] 



334. What are the Node Months of a 
The months in which the Earth passes the line of its 

nodes, and in which all its transits must occur. 

[As the Earth's revolution around the Sun constitutes its year, and the two nodes of 
a planet are in opposite sides of the ecliptic, it follows that the Earth must pass the 
nodes in opposite months in the year.] 

335. Which are the Node Months of Mercury ? 



PRIMARY ASTRONOMY. 75 



May and November. 



[All the transits of Mercury ever noticed have occurred in one or the other of these 
months, and for the reason already assigned. The first ever observed took place 
November 6, 1631 ; since which time there have been 29 others by the same planet— 
in all 30—8 in May, and 22 in November.] 

336. When did the last transit of Mercury take place ? 
November 9, 1848. 

[This transit was observed by Professor Mitchel at the Cincinnati Observatory, and 
by many others in America and in Europe. The writer had made all necessary 
preparation for observing the phenomenon at his residence, near Oswego, New York; 
but, unfortunately, his sky was overhung with clouds, which hid the Sun from his 
view, and disappointed all his hopes.] 

337. When will the next occur 1 
November 11, 1861. 

[There are to be five besides this during the present century ; two in May, and three 
in November.] 

33§. Where does the line of Venus' 's Nodes lie ? 

In Gemini and Sagittarius. 

[As the planets all revolve in the same direction, it will be seen, by consulting the last 
figure, that Venus's ascending node must be in JJ and her descending node in t 1 

339. Which are Venus's Node Months 1 
December and Jnne. 

340. Why is this ? 

Because the Earth always passes Gemini and Sagit- 
tarius during these months. 

[This will be obvious by consulting the last cut; for as the line of Venus's nodes is 
only one sign ahead of that of Mercury, the Earth will reach that point in the ecliptic 
in one month after she passes the line of Mercury's nodes; so that if his transits 
occur in May and November, here should occur in June and December, as is always 
the case.] 

341. Wlien did the last transit of Venus occur ? 

December 6, 1822. 

342. When will the next take place ? 
December 8, 1874. 

[1. Only four transits of Venus have as yet been observed ; namely, December 4, 
1639 ; June 5, 1761 ; June 3, 1769; and December 6, 1822. It is said that Rittenhouse 
was so interested in viewing that of 1769, that he actually fainted. In defining the 
term transit, Dr. Webster says: "I witnessed the transit of Venus over the Sun's 
disk, June 3, 1769." See " Unabridged" Dictionary. 

2. The next four will occur December 8, 1874 ; December 5. 1882 ; June 7, 2004 ; 
and June 5, 2012.1 



76 



PEIMAEY ASTRONOMY. 



LESSON XIX. 

THE SUN'S APPARENT MOTIONS, THE SEASONS, ETC. 
343* What are the Sun's apparent motions 1 
He appears to revolve daily from east to west around 
the globe. 

344. By what is this apparent motion produced ? 

[For an answer let the student consult his memory ; but if this fails, he may turn 
back to Question 14.] 

345. What other apparent motion has the Sun ? 

He appears to travel eastward through all the signs of 
the zodiac every 365^ days. 

346. To what is this apparent motion attributable ? 

To the revolution of the Earth in her orbit around the 
Sun. 

sun's apparent motion around the ecliptic. 




PRIMARY ASTRONOMY. 77 

[Suppose the Earth is at A on the 20th of March ; the Sun will appear to he at B in 
the opposite side of the ecliptic. As the Earth moves on in her orbit from A to C, 
the Sun will appear to move from B to D ; and will seem thus to traverse the whole 
circle of the heavens every 365^ days, or as often as the Earth revolves around 
him.] 

347. What is the cause of the different seasons of the year — as 
Spring, Summer, c^c. ? 

It is owing to the inclination of the Earth's axis to 
the ecliptic, and her revolntion around the Sun. 
34§. How much is the Earth? s Axis inclined to the Ecliptic ? 
Twenty-three degrees and twenty-eight minutes. 

INCLINATION OF THE EARTH'S AXIS TO THE PLANE OF THE ECLIPTIC. 

THE. ECLIPTIC /<&£ 



349. Does the Earth's Axis always incline the same way and to 
the same amount ? 

It does. 

[1. This general fact is shown in the cuts, pages 65 and 76, where her axis is seen 
parallel with itself, or pointing in the same direction, in every part of her orbit. 

2. The author is aware that the poles of the Earth have a slow motion around the 
pole of the ecliptic, requiring 25,000 years for a single revolution ; but regarding this 
and the Precession of the Equinoxes as difficult matters to the juvenile comprehension, 
and not essential to an explanation of the seasons, &c, has designedly omitted them 
both.] 

350. What effect has this inclination and permanency of the Earth's 
Axis in the production of the Seasons? 

As the Earth revolves around the Sun, it brings first 
one pole toward the Sun and then the other; thus 
causing a constant variation of light and heat, and pro- 
ducing the seasons. 

[1. This may be illustrated by reference to the opposite cut, where the Earth is 
shown at four different points, with her axis inclined to the left 23° 28'. 

2. The student will readily see that if a planet whose axis is thus inclined revolves 
around the Sun, it must give one hemisphere the most light and heat for half its 
year, and the other during the remainder. For instance, from A around to B in the 
cut, the hemisphere toward us would have the most light ; while from B around to C 
again, the opposite one would have the most. This variation in the light and heat re- 
ceived from the Sun at different points on the Earth, during certain months in the year, 
is what causes the Seasons. 

3. Many very intelligent people in other respects, have an idea that the Earth's axis 



78 



PRIMARY ASTRONOMY. 



"wabbles," as they call it, so as to bring first the Northern and then the Southern 
Hemisphere toward the Sun, and produce the Seasons; but the permanency of the 
axis effects all this by the simple revolution of the Earth in her orbit.] 

351. Is the Earth's orbit a Circle or an Ellipse ? 

It is elliptical, like all the planetary orbits. 

352. Where do her Perihelion and Aphelion points lie? 

Her Perihelion is in Gemini, and her Aphelion in 
Sagittarius. 

353. When is the Earth nearest to the Sun, or at Perihelion ? 
About the first of January. 

354. When is she at her greatest distance 1 
Six months afterward, or July 3. 

355. How is it, then, that it is cold in January, when the Sun 
is nearest ; and warm in July, when he 
is most distant! 

It is because the Northern 
Hemisphere inclines toward the 
Sun in July, and from him in 
January. 

[The comparative amount of light received in 
the Northern Hemisphere in July and January, 
may be illustrated by the accompanying figure, 

in which the rays of light at different seasons are represented to the eye. Tn January 
they are seen to strike the Northern Hemisphere obliquely; and consequently the 
same amount of light is spread over a much greater surface. In July the rays fall 
almost perpendicularly upon us, and are much more intense. Hence the variations of 
temperature which constitute the Seasons.] 



SUMMER AND WINTER RAYS. 




356. What is the difference of the Earth's distance in July and 
January ? 

About three millions of miles. 

357. Does not this in reality affect the general temperature of 
the Earth 7 

It undoubtedly does ; but the variation of 3,000,000 
miles is so slight when compared with the whole dis- 
tance of the Sun, that the change of temperature pro- 
duced thereby is imperceptible. 



PHIMARY ASTRONOMY. 79 

[The natural effect of this variation would be, so far as it had any influence, to 
modify the cold and heat in the Northern Hemisphere, and to augment both in the 
Southern. For instance, our nearness to the Sun in January would slightly soften our 
Winter, while, at the same time, it slightly increased the heat of the Summer south of 
the equator. So, also, our increased distance in July would diminish the heat of our 
Summer, and at the same time enhance the cold of the corresponding Winter in the 
Southern Hemisphere.] 

35 §. What are the Equinoctial Points in the Earth's orbit ? 

They are two points, 180° apart, where the Sun shines 
perpendicularly upon the equator; or, in other words, 
is m the Equinoctial. 

[1. The Earth is shown at these points in the cut, page 76. See A and B. 

2. If the Sun is vertical at the equator, he will, of course, shine to both poles, as 
represented in the cut, and the days and nights will be equal all over the world. 
Hence the name equinoctial, from the Latin cequus, equal, and nox, night.] 

359, How are the Equinoctial Points distinguished? 
Into Yernal and Autumnal. 

380. Why this distinction ? 

Because the Earth passes one on the 20th of March, 
when the Sun crosses the equator northward, and Spring 
begins ; and the other on the 23d of September, when 
the Sun passes south of the equinoctial, and Autumn 
begins. (See cut, page 76.) 

361. What effect has the inclination of the Earth's Axis and 
her Annual Revolution upon the apparent motion of the Sun ? 

It causes him to appear to change his path in the 
heavens, coming up north nearly overhead in the Sum- 
mer, and dropping low down in the south in the 
Winter. 

362. What is this variation of the Sun north and south of the 
Equator called? 

His declination. (See 116.) 

363. How far does the Sun decline north and south of the 
Equator ? 

Only 23° 28' on each side, or as far as the Tropics. 
(See 97 and 98, and Notes.) 



80 



PEIMAET ASTKONOMY. 



sun's declination. 










[1. The apparent position of the Sun with re- 
spect to the equator, at different seasons of the 
year, may be illustrated by the annexed cut. On 
the 21st of June he has his greatest northern dec- 
lination, or Summer Solstice, and is vertical on 
the Tropic of Cancer. From this time he ap- 
proaches the equator of the heavens till the 20th 
of September, when he crosses it, and begins to 
decline southward. On the 23d of December he 
has reached his greatest southern declination, or 
Winter Solstice, and begins to return toward the 
equinoctial, which he passes on the 20th of 
March, and reaches his Summer Solstice again 
on the 21st of June. In this manner he continues 
to decline, first north and then south of the 
equator, from year to year. Tf the student does 
not fully understand the cause of the Sun's declination, let him turn back and con- 
sider 347, and carefully observe the cut on the same page. 

2. The Sun's declination may be easily measured shadows at the equator. 
by the shadow of a suitable object upon the Earth's 
surface. Suppose the flag-staff in. the cut to stand 
perpendicularly, and exactly on the equator. On 
the 23d of December the shadow would be thrown 
northward to A, or 23° 28'— just as far as the Sun 
has declined south. At 12 o'clock, on the 20th of 
March, and the 23d of September, there would be 
no shadow ; and on the 21st of June it would ex- 
tend southward 23° 28' to C. Thus, at the equa- 
tor, the shadow falls first north and then south of 
all perpendicular objects, for six months alter- 
nately. 




MEASURING THE SUN S DECLINATION IN NORTHERN LATITUDE. 




3. This cut shows how the student may measure the Sun's declination wherever he 
may be located north of the equator. The shadows are such as are cast by objects 
during the year, about 45° north of the equator. On the 23d of December, when the 
Sun has his greatest declination, the shadow of the flag-staff extends north at 12 
o'clock to the point C, where two boys are seen, having just driven down a stake. 
From this time to June 21st the shadow gradually shortens, till on that day it reaches 
the point B, where another stake is driven. It then begins to elongate, and in six 
months is extended to C again. The point A is just half way from B to C in angular 
measurement, though the distances on the plain in the picture are very different. 



PEIMARY ASTRONOMY. 81 



When the Sun is on the equator, March 21st and September 23d, the shadow will 
reach only to A; and the angle A B and the top of the staff shows the northern, and 
A C and the top of the staff the southern declination. It will be found to be 23° 28' 
each way, as marked in the figure. 

4. The angle formed by the top and bottom of the pole and the point A will 
exactly correspond with the latitude of the place where the experiment is made. 

5. Let the students try this matter for themselves. Select a level spot, and put up a 
stake, say ten feet high. Get an exact " noon mark," or north and south line, where 
the stake is driven, and at 12 o'clock, every fair day, put down a small stake at the end 
of the shadow. In this manner you will soon be able to measure the Sun's declina- 
tion for yourselves ; to determine the latitude of the place where you live ; and to un- 
derstand how mariners at sea ascertain their latitude by the declination of the Sun. 

6. The ancients had pillars erected for the purpose of making observations upon 
their shadows. Such a pillar is called a gnomon.} 

364. What are the Solstitial Points? 

They are those points in the Earth's orbit where the 
Sun ceases to decline north or south, and begins to 
return toward the equinoctial. 

[See the points E and F in the cut, page 76.] 

365. How are the Solstices distinguished? 
Into Summer and Winter. 

366. Why are they thus distinguished? 

Because the Earth passes one on the 23d of Decem- 
ber, or in the Winter ; and the other on the 21st of 
June, or in the Summer. 

[By a little attention, the student may observe that the Sun is most directly over- 
head in the Northern Hemisphere on the 21st of June, and lowest in the south on the 
23d of December. From June 21st to December 23d he appears to go southward, while 
from December 23d to June 21st he comes northward ; thus producing the agreeable 
changes and the unnumbered blessings of the seasons.] 

367. What is meant by the Obliquity of the Ecliptic ? 

It is the angle which the equinoctial makes to the 
ecliptic, in consequence of the inclination of the Earth's 

£,XiS. OBLIQUITY OF THE ECLIPTIC. 

[Here it will be seen that in the same 
proportion that the axis of the Earth 
inclines from a perpendicular toward 
the ecliptic the equator of the Earth 
must depart from the plane of the eclip- 
tic ; and as the axis inclines 23° 28', the 
equinoctial departs 23° 28'. This an- 
gle, thus formed, shown at A and B, 
constitutes the obliquity of the ecliptic] 




82 



PEIMAEY ASTEONOMY. 



3©§. How are the Axes of the other planets inclined to the Planes 
of their respective Orbits ? 



Mercury unknown. 

Venus 75° 00' 

Mars 28 40 



Jupiter 3° 5' 

Saturn 30 

Herschel and Neptune unknown. 



INCLINATION OF THE AXES OF THE SEVERAL PLANETS TO THE PLANES OF 
THEIR ORBITS. 




[The student must bear in mind, that the inclination above represented is not to the 
ecliptic (with the exception of the Earth), but to the planes of their several orbits ; for 
instance : 

Venus. 



'T>j 




PLANE OF VEN^li^I 



PLANE OF THE ECLIPTIC 



The orbit of Venus departs from the ecliptic 3i°, as stated at 325, while her axis is in- 
clined to the plane of her orbit 75°, as shown in the above figures. This distinction 
should be kept definitely in view by the student.] 



LESSOIST XX. 



SEASONS OF THE DIFFERENT PLANETS, TELESCOPIC VIEWS, ETC. 

369. What influence has the inclination of a planet's axis upon its 
Seasons ? 

It determines the extent of its zones; or, in other 
words, the amount of the Sun's declination north and 
south of its equator. 

[If this is not clear to the mind of the student, let him consult the first of the above 
diagrams, from which it will be obvious that the less the inclination the narrower the 
Torrid Zone, and the smaller the Polar Circles.] 



PKIMARY ASTRONOMY. 83 

370. What influence has the Periodic Time of a planet upon its 
Seasons 1 

It determines their length. 

[As the axes of the several planets are permanent, they can have but four regular 
seasons in their year, however long it may be.] 

371. What can you say of the Seasons of Venus? 

Her Tropics are within 15° of her Poles, making her 
Torrid Zone 150° wide. The Sun passes from one 
Tropic to the other and back in 225 days, during which 
time she has her four seasons of 56 J days each. 

372. Describe the Seasons of Mars. 

They are much the same as those of our Earth, ex- 
cept that they are longer. 

[As the year of Mars consists of 687 days, his four seasons must consist of 172 days 
each, oi- nearly twice the length of the seasons of the Earth.] 

373.. Has Jupiter any change of Seasons ? 

Scarcely any. His axis being inclined to his orbit 
only 3° 5', the Sun never departs more than 3° 5' from 
his equator. 

374. What effect does that have ? 

It causes perpetual summer at his equator, perpetual 
winter at his poles, and gives the intermediate regions 
an almost unchangeable temperature. 

375. By what are the Seasons of Saturn distinguished! 

His zones are much like those of our globe, but each 
of his seasons is about Ti years long. 

[He has four seasons in his periodic time, the same as the Earth and other planets ; 
and as that is about 30 years, each season must consist of about 7| years.] 



376. How long are his Poles alternately in tlie light and 
For about fifteen years. 

377. Have we any knowledge of the Seasons of Herschel and 
Neptune ? 

ISTone, except that, being very remote from the Sun, 
their general temperature must be very low. 



84 PRIMARY ASTRONOMY. 

3Y8. Are all the Primary Planets visible to the naked eye 1 

They are not. 

379. Which of them can be thus seen 7 

Mercury, Yenus, Mars, Yesta, Jupiter, and Saturn. 

[It is stated upon pretty good authority, that Herschel has been seen, under very 
favorable circumstances, as a star of the sixth or seventh magnitude ; but, as a general 
thing, he is invisible, except by the aid of the telescope.] 

3§0. How does Mercury appear to the naked eye 1 

As a star, always in the neighborhood of the Sun, 

381. How does he look through a Telescope ? 

Like a globe or world, with numerous spots upon its 
surface. 

3§2. What are these Spots supposed to be? 

The natural divisions of the planet— as Continents, 
Islands, Mountains, &c. 

[Schroeter, an eminent German astronomer, measured several mountains upo» the 
surface of this planet, one of which he found to be nearly eleven miles in hightj 

3§3. What is the general Color and appearance of Mercury ? 

Through a telescope he has a faint "bluish tint ? and 
exhibits a great variety of forms or appearances. 

384. What is the natural appearance of Venus? 

Her color is of a silvery white ; and when at a dis- 
tance from the Sun, either east or west, she is exceed- 
ing bright and beautiful. 

3 §5. How does she appear through a Telescope ? 

As she passes around the Sun she exhibits all the 
varying phases of the Moon, 

TELESCOPIC PHASES OF VENUS. 

fj EAST OF THE SUN M^h WEST OFTHE SUM ^'-w 

^<^^ AND "EVENING STAR £0^ AND MOR NtNfr STAR fB W 



PRIMARY ASTRONOMY. 



85 



[1. The telescopic appearance of Venus, at different points in her orbit, is represented 
in the last figure. At E and W she has her greatest eastern and western elongation, 
and is stationary ; while her positions opposite the words " direct" and " retrograde" 
represent her at her conjunctions. The spots on the face of the Sun represent Venus 
projected upon his disk, in a transit, the arrow indicating her direction. 

2. Before the discovery of the telescope it was asserted, that if the Copernican theory 
were true, Mercury and Venus would exhibit different phases at different times; and as 
those phases could not be seen, it was evident that the theory was false. But no 
sooner had Galileo directed his small telescopes to these objects, than he found them 
exhibiting the very appearances required by the Copernican theory, its opponents 
themselves being judges.] 

386. Explain the cause of the different Phases of Mercury and 
Venus. 

It is because we see more of their enlightened sides 
at one time than at another. 

387. What else does the Telescope reveal upon Venus ? 

A variety of spots, probably Islands, Continents, and 
Seas. 

SPOTS SEEN UPON THE PISK. OF VENDS. 




388. Has Venus any Mountains? 

She has ; some of which are supposed to be over 
twenty miles in hight. 

[Three elevations upon her surface have been estimated at 10$, 11£, and 19 miles, 
respectively.] 

389. What can you say of her Atmosphere ? 

It is supposed to be very dense, and to surround the 
planet only to the depth of about three miles. 

[The atmosphere of our own globe is supposed to extend about forty miles from its 
surface, or thirteen times as far as that of Venus.] 

390. Why is it thought that the Spots seen upon Mercury and 
Venus are the great natural divisions of their surfaces ? 

Because such divisions would appear like spots, if 
viewed from a distance, and would vary as the planets 



86 



PRIMABY ASTRONOMY. 



revolved, precisely as the spots vary upon Mercury and 
Yenus. 

391. How would the Continents, Islands, and Seas of our Globe 
appear at the distance of Mercury and Venus 1 

As mere spots upon its surface, resembling those seen 
upon those planets. 

DISTANT TELESCOPIC VIEWS OF THE EARTH. 
1. 2. 3. 




[Above we have four different views of our own globe. No. 1 is a view of the 
Northern Hemisphere ; No. 2, of the Southern ; No. 3, of the Eastern Continent ; No. 4, 
of the Western. A common terrestrial globe will present a different aspect from every 
new position from which it is viewed ; as the Earth must in her appearance to the in- 
habitants of other worlds.] 

392. How does Mars appear to the naked eye! 
Like a bright star of a reddish color. 

[Just east of the "Seven Stars," or Pleiades, the student will find another group 
called the Hyades ; one of which, called Aldebaran, is of a reddish cast, and somewhat 
resembles the planet Mars. When Mars is in opposition, however, at his nearest 
point to us, and with his enlightened side toward us, he appears much larger and 
brighter than Aldebaran. See the position of Mars when in opposition, as illustrated 
by the cut, page 50.] 

393. How does he appear through a Telescope 1 

He has a reddish hue, and exhibits slight phases, and 
a variety of spots upon his disk. 

394. What is supposed to be the cause of the peculiar Color of 
Mars ? 

It is attributed to his extended and very dense atmos- 
phere. 

[When the sunlight passes through vapor or clouds in the morning or evening, the 
different rays of which it is composed are separated, and the red rays only pass to the 
Earth, giving to the clouds a gorgeous crimson appearance. In a similar manner it is 
supposed that the atmosphere of Mars may give him his crimson hue.] 

395. What do astronomers think of the Spots upon his surface ? 



PREtfARY ASTRONOMY. 



87 



" Upon this planet," says Dr. Herschel, " we discern, 
with perfect distinctness, the outlines of what may be 
Continents and Seas." 

396. What peculiar changes are seen to take place about his 
Poles 1 

When it is Winter at his north pole, that part of the 
planet is white, as if covered with ice and snow ; bnt as 
Summer returns to his Northern Hemisphere, the bright- 
ness about his north pole disappears. 

TELESCOPIC APPEAKAXCES OF MARS. 




[The right-hand figure represents Mars as seen at the Cincinnati Observatory, 
August 5, 1845. On the 30th of the same month he appeared as represented on the 
left. The middle view is from a drawing by Dr. Dick.] 



LESSOR XXI 



TELESCOPIC YTEWS OF THE PLANETS CONTINUED. 

397. What peculiarities do the Asteroids present under the Tele- 
scope ? 

A thin haze is seen around Pallas ; and they are all 
of a pale ash-color, except Ceres, whose color is like that 
of Mars. 

39§. Describe the Telescopic appearance of Jltiter. 

His form is seen to be oblate ; his color a light yel- 
low ; and his disk is streaked with several curious 
belts. 



88 



PRIMARY ASTRONOMY. 



399. How are these Belts situated 1 

On both sides of his equator, and parallel to it. 

400. What is their number ? 
Only two or three are 

generally seen, though 



TELESCOPIC VIEW OF JUPITER. 



more are sometimes vis- 
ible. 

[1. Much depends upon the power 
of the instrument through which he 
is viewed. An ordinary telescope 
will show the two main belts, one 
each side of his equator ; but those 
of greater power exhibit more of 
these curious appendages. Dr. Her- 
schel once saw his whole disk cov- 
ered with small belts. 

2. The preceding cut represents 
Jupiter as seen through the great 
Refracting Telescope at Cincinnati. 
It is copied from the Sidereal Mes- 
senger of February, 1847.] 




401. Do these Belts appear permanent or fluctuating ? 

They sometimes continue without change for months, 
and at other times break up and change their forms in 
a few hours. 

402. Are they regular or otherwise 1 

They are quite irregular, both in form and apparent 
density ; as both bright and dark spots appear in them, 
and their edges are always broken and uneven. 

[The preceding cut affords a good idea of the appearance of these belts, and the 
spots seen in them.] 

403. What are these Belts supposed to he ? 

They are thought to be openings in the atmosphere 
through which the body of the planet is seen. 

[The rapid motion of Jupiter upon his axis is supposed to throw the clouds which 
float in his atmosphere into parallel strata, leaving regular interstices between them, 
through which the opake body of the planet is seen.] 

404. How are the Spots in these Belts accounted for ? 

They are supposed to be caverns, mountains, or some- 



PRIMARY ASTRONOMY. 89 

thing unknown to us, but permanently attached to the 
body of the planet. 

[One of these spots, first observed in 1665, disappeared, and reappeared in the same 
form for more than forty years ; showing conclusively that it was something perma- 
nent, and not a mere atmospherical phenomenon.] 

405. What else do we notice in examining Jupiter through a 
Telescope 1 

Four small stars are seen near him, and revolving 
around him. 

[1. These are the satellites of Jupiter, of which we shall give a more particular 
account when we come to speak of the Secondary Planets. 

2. The writer once saw all four of these satellites at once, and very distinctly, through 
a common ship telescope, worth only twelve or fifteen dollars. They were first seen 
by Galileo with a telescope, the object-glass of which was only one inch in diameter ! 
If the student can get hold of any such instrument whatever, let him try it upon Jupi- 
ter, and see if he cannot see from one to four small stars near him, that will occupy 
different positions at different times.] 

406. How does the body of Saturn appear through a Telescope ? 

Like an oblate globe, of a lead color, striped with 
belts, like those of Jupiter. 

[The oblateness of Saturn is really greater than that of Jupiter (Question 281) ; but 
as he is more remote than the latter planet, the depression at his poles, &c, is ren- 
dered less distinct.] 

407. What remarkable appendage is connected with this Planet 1 

He is surrounded by two wonderful Rings of a silvery 
white color. 

408. How are they situated with reference to the planet, and to each 
other ? 

They are directly over his equator, the first about 
20,000 miles from his surface, and 20,000 miles wide. 
There is then an opening of 2,000 miles, when we come 
to the exterior «ring, which is 10,000 miles wide. 

409. How is it known that these Rings are separate from the body 
of the planet and from each other ? 

From the fact that the fixed stars, in the heavens 
beyond, have been seen through the openings between 
them. 



8* 



90 



PRIMARY ASTRONOMY. 



TELESCOPIC VIEW OF SATURN. 




PERPENDICULAR VIEW OF TUI 
RINGS OF SATURN. 



[1. The writer has often 
seen the opening between 
the body of the planet and 
the interior ring, as distinct- 
ly as it appears to the stu- 
dent in the adjoining cut. 

2. This is an oblique view 
of the rings, and about the 
best that can be obtained. 
It represents the planet as 
seen at the Cincinnati Ob- 
servatory, November, 1846. 

3. We sometimes see the 
planet when the edge of the 

rings is turned toward us, but we never get a perpen- 
dicular view of them. Could the planet be seen from a 
point over either of his poles, the rings would doubtless 
appear as represented in this second figure. 

4. Under very powerful telescopes, these rings are 
found to be again subdivided into an indefinite num- 
ber of concentric circles, one within the other.] 

410. What is the thickness of these 
Rings ? 

It is estimated at about 100 miles. 

411. Are they supposed to be solid, like 
the body of the planet ? 

They are; from the fact that they sometimes cast a 
strong shadow themselves upon the body of the planet ; 
and at other times show the planet's shadow very dis- 
tinctly upon their own surfaces. 

412. Are they at rest or in motion ? 

They revolve eastward around the planet every 10^ 
hours ; or in the time of his rotation upon his axis. 

[This revolution resembles that of the rim of a carriage-wheel around the hub, 
except that there are no spokes in the case of Saturn to unite the center to the circum- 
ference. This defect, however, is perfectly supplied by the law of gravitation.] 

413. What changes are seen to take place in the appearance of 
the Rings during the planets revolution around the Sun ? 

The apparent ellipse of the rings seems to contract for 
about 7|- years, till it almost entirely disappears, when 
it begins to expand again, and continues to enlarge for 
7J years. 




PRIMARY ASTRONOMY. 



91 



414. What other change has been noticed? 

For fifteen years the part of the rings toward us 
seems to be thrown up, while for the next fifteen it ap- 
pears to drop oelow the apparent center of the planet. 

TELESCOPIC PHASES OF THE RINGS OF SATURN. 



•« Jf <q»2>X 



[The cause of these varying appearances of Saturn will be easily understood by ex- 
amining the next cut and the accompanying notes.] 

415. How are these Rings affected as respects Light and Shade ? 

The Sun shines, alternately, fifteen years upon one 

side, and fifteen upon the other. 

SATURN AT DIFFERENT POINTS IN HIS ORBIT. 




[1. Here observe, first, that tlie axis of Saturn, like those of all the other planets, 
remains permanent, or parallel with itself; and as the rings are in the plane of his 
equator, and at right angles with his axis, they also must remain parallel to them- 
selves, whatever position the planet may occupy in its orbit. 

2. This being the case, it is obvious that while the planet is passing from A to E, the 
Sim will shine upon the under or south side of the rings ; and while he passes from E 
to A again, upon the upper or north side ; and as it requires about 30 years for the 
planet to traverse these two semicircles, it is plain that the alternate day and night 
on the rings will be 15 years each. 

3. A and E are the equinoctial end C and G the solstitial points in the orbit of 
Saturn. At A and E the rings are edgewise toward the Sun, and also toward the 
Earth, provided Saturn is in opposition to the Sun. The rings of Saturn were invisi- 
ble as rings from the 22d of April, 1848, to the 19th of January, 1849. He came to his 
equinox September 7, 1848, from which time to February, 1856, his rings will con- 
tinue to expand. From that time to June, 1863, they will contract, when he will reach 



92 PRIMARY ASTRONOMY. 



his other equinox at E, and the rings will he invisible. From June, 1863, to Septem- 
ber, 1870, they will again expand ; and from September, 1870, to March, 1877, they 
will contract, when he will be at the equinox passed September 7, 1848, or 29£ years 
before. 

4. This cut will illustrate Questions 413 and 414. To an observer on the Earth the 
rings will seem to expand from A to C, and to contract from C to E. So, also, from E 
to G and from G to A. Again : from A to E the front of the rings will appear above 
the planet's center, and from E to A below it. 

5. The writer has often seen the rings of Saturn in different stages of expansion and 
contraction, and once when they were almost directly edgewise toward the Earth. At 
that time (January, 1849,) they appeared as a bright line of light, as represented at 
A and E, after 414.] 

416. What purposes do these Rings serve, as appendages to 
Saturn ? 

They reflect the sunlight upon his surface, as our 
Moon does upon the surface of the Earth. 

417. How must they appear to a person upon the body of the 
planet, either north or south of his Equator ? 

Like two gorgeous arches of light, bright as the full 
Moon, and spanning the whole heavens from east to 
west. 

[In the annexed cut, the beholder is sup- 
posed to be situated some 30° north of the 
equator of Saturn, and looking directly 
south. The shadow of the planet is seen 
traveling up the arch as the night ad- 
vances, while a New Moon is shown in the 
west, and a Full Moon in the east at the 
same time.] 



NIGHT SCENE UPON SATURN. 




418. How does the width of the two Rings compare ivith the 
diameter of the Moon ? 

The two rings united are nearly 13 times as wide as 
the diameter of the Moon. 

[The two rings are 30,000 miles wide, which, being divided by 2,160, the diameter 
of the Moon, gives 121 as the result.] 

419. How does their distance from Saturn compare with that of 
the Moon from our globe 1 

The nearest ring is only one-twelfth pevrt as far from 
the planet as our Moon is from us. 

[I. Divide 240,000 miles, the Moon's distance, by 20,000, the distance of the nearest 
ring, and we have the above result. 
2. At the distance of only 20,000 miles, our Moon would appear some forty times as 



PEIMAEY ASTRONOMY. 93 

large as she does at her present distance. How magnificent and inconceivably grand, 
then, must these vast rings appear, with a thousand times the Moon's magnitude, and 
only one-twelfth part of her distance !] 

420. What else does the Telescope reveal in connection with 
Saturn ? 

Eight small stars are seen in his vicinity, which are 
found to be Moons revolving around him. 

[These are seen only with good instruments. On one occasion the writer saw five of 
them at once through the instrument represented in the frontispiece ; but the remain- 
ing three he has never seen.] 

421. How does Herschel appear through a Telescope? 

Like a small ash-colored globe, without rings, belts, 
or discernible spots. 

[Of his six Moons we shall speak in another lesson.] 

422. What can you say of the Telescopic appearance of Nep- 
tune? 

In color and general appearance he resembles Her- 
schel. 

[So far as is known, Neptune has no rings nor belts, and is attended by only one 
Moon.] 



LESSON XXII. 

OF THE SECONDARY PLANETS. 

How are the planets of our system divided (133) ? 
What are the Primary Planets (134) % 
Describe the Secondary Planets (135). 

Which of these classes have you been considering in the last fourteen 
lessons ? 

423. How many Secondary Planets are there now known to be? 

Twenty. 

424. How are they distributed among the Primaries ? 

The Earth has one, Jupiter four, Saturn eight, Her- 
schel six, and Neptune one. 

425. By what other names are the Secondary Planets known ? 



94 PEIMAEY ASTRONOMY. 

They are often called Moons or Satellites. (See ]STote 
to 135.) 

426c How are they situated with reference to their respective 
Primaries ? 

They are placed at different distances ; as the Prima- 
ries are placed with respect to the Sun. 

427. What can you say of their motions ? 

They revolve aronnd their respective Primaries, from 
east to west, and at the same time accompanying them 
around the Sun. 

[The Moons of Herschel are said to be an exception to this remark, and to revolve 
backward or westward, unlike any other bodies in the Solar System.] 

42 §. How are their Orbits generally situated? 
In or near the plane of the equators of their respect- 
ive Primaries. 

[Herschel is supposed to be an exception to this rule also : the orbits of his satellites 
lying almost at right angles with the plane of his orbit. It may be, however, that his 
axis is nearly parallel with the plane of his orbit, as is the case with Venus ; and that 
his Moons are, after all, in the plane of his equator.] 



THE MOON -HER DISTANCE, MAGNITUDE, ETC. 

429. By what name was the Moon known to the ancients ? 

The Romans called her Luna, and the Greeks Selene. 

[1. From Luna we have our modern terms lunar and lunacy ; the former of which 
signifies pertaining to the Moon, and the latter a disease anciently supposed to be 
caused by the Moon. 

2. Selene, in Mythology, was the daughter of Helios, the Sun. Our English word 
selenography— a description of the Moon's surface— is from Selene, her ancient name, 
and grapho, to describe.] 

430. How has the Moon generally been regarded by mankind ? 
As the most interesting object in the heavens. 

[Pier beauty has been celebrated in the poetry of every age.] 

431. Why has she attracted so much attention? 

On account of her remarkable changes both of position 
and appearance. 



PEIMAEY ASTEONOMY. 95 

432. How is she situated with respect to the Earth ? 
She is the nearest of all the heavenly bodies. 

433. What is her average distance ? 

About 240,000 miles. 

434. What is her Magnitude ? 

Her diameter is 2,160 miles. 

435. How, then, does her bulk compare with that of the Earth ? 
She is only -j^th part as large. 

[The masses of globes are in proportion to the cubes of their diameters. Then 
2,160 X 2,160 X 2,160 = 10,077,696,000, the cube of the Moon's diameter ; and 7,912 
X 7,912 X 7,912 = 495,289,174,428, the cube of the Earth's diameter. Divide the 
latter by the former, and we have 49 and a fraction over, as the number of times the 
bulk of the Moon is contained in the Earth.] 

436. How does her diameter compare with that of the Sun ? 
It is only abont 4-jyoth part as great. 



[886,000 — 2,160 = 401 5, the number of times the Moon's diameter fifW" 
is contained in that of the Sun.] >%j 1W; 

437. What is the apparent diameter of the Moon? \ j 

Thirty-one degrees and seven minutes (31° 1'). I j 

43§. How do the Sun and Moon compare in their ap- i j 

parent magnitudes ? \ \ 

They appear about of a size. j j 

[As the mean angular diameter of the Sun is 32' 2", and that of j i 

the Moon 31' 7", the difference can only be 55". Both the Sun and i j 

Moon vary in their apparent magnitudes, as their distances vary.] : j 

439. What is tlieir real comparative bulk? \ j 
The Sun is seventy mAllion times as large j i 

as the Moon. I j 

440. How is it, then, that they appear so near of a 

size? i : 

It is because the Sun is 400 times as far Ij 

off as the Moon. Jj 

[The adjoining cut shows that small as the Moon is, she fills as large / J^N\ 
an angle at A as the Sun does at B.] ■ Igjr ; 



96 PRIMARY ASTRONOMY. 



LESSON XXIII. 

REVOLUTION OF THE MOON ABOUND THE EARTH. 

441. In what direction does the Moon revolve around the 
Earth 1 } 

From west to east. 

442. Is that her apparent daily course in the heavens ? 
It is not. 

443. What causes her apparent daily revolution westward ? 
The revolution of the Earth eastward upon its axis. 

(See Question 28, and Notes.) 

444. What proof have we that the Moon actually revolves east- 
ward ? 

By watching her for a single evening, we can per- 
ceive that while she seems to go over us westward, she 
is actually moving eastward among the stars. 

445. Have we any other proof? 

At New Moon she is near the Sun in the west, and 
continues to separate from him till Full Moon, when 
she is in the east From that time she approaches the 
Sun, till she meets and passes him from the west. 

446. What is the Periodic Time of the Moon? 

Her Sidereal devolution is performed in 27^ days. 

447. What is meant by her Sidereal Revolution? 

It is a revolution from any given point in her orbit 
around to the same point again. (See 234, and Notes.) 

448. What is a Synodic Revolution of the Moon ? 

It is from one new Moon, or conjunction with the 
Sun, to another. 

449. Can you state the difference between a Sidereal and a Sy- 
nodic Revolution of the Moon, and explain it by a diagram ? 

The Sidereal is one complete revolution; but the 



PEIMAEY ASTRONOMY. 



97 



motion of the Earth in her orbit renders it necessary 
for the Moon to perform a little more than a complete 
revolution each month, in order to come in conjunction 
with the Sun, and make a Synodic revolution. 

SIDEREAL AND SYNODIC REVOLUTIONS OF THE MOON. 



SIDEREAL REVOLUTION 2.7JDAYS 



ir 



cyN 



oo\c. 



*^.°. 



z\MV..-- 



^^\' \\ . 



v*i 



o 



SUN AND MOON IN C ONJ DNOT I N- NEW MOON,' 



<w^ 



VJj 



[I. On the right the Earth is shown in her orhit, revolving around the Sun, and the 
Moon in her orbit, revolving around the Earth. At A the Sun and Moon are in con- 
junction, or it is J\Tew Moon. As the Earth advances from D to E, the Moon passes 
around from A to B, or the exact point in her orbit where she was 27£ days before. 
But she is still west of the Sun, and must pass on from B to C, or 1 day and 20 hours 
longer, before she can again come in conjunction with him. This 1 day and 20 hours 
constitutes the difference between a Sidereal and a Synodic revolution. 

2. The student will perceive that the difference between a Sidereal and Synodic 
revolution of the Moon, like that between Solar and Sidereal time, is due to the same 
cause ; namely, the revolution of the Earth around the Sun. See Question 267, with 
the Illustration and Notes.] 

450. What is the daily an- ™ily progress of th. moon eastward. 
gular motion of the Moon east- 
ward ? 

Thirteen degrees, ten 
minutes, and thirty -five 
seconds (13° 10' 35"). 

[This estimate is made for a Solar day 
of twenty-four hours. In the adjoining 
cut the daily progress of the Moon may 
be traced from her conjunction or 
" change"" at A on the right, around to 
the same point again. This being a 
Sidereal revolution, requires only 27£ 
days.] 

451. What is the Moon's Velocity in her orbit, with respect to 
the Earth ? 

About 2,300 miles per hour. 




98 



PRIMARY ASTRONOMY. 



452. What is the Form of her Orbit ? 
With respect to the Earth it is elliptical, like those 
of all the other planets. 



APOGEE. 

..-€-.. 



/ 



••••-©•- 
PERIGEE. 



453. When is the Moon said to be in 
Perigee 1 

When in that part of her orbit 
nearest to the Earth. 

[The word Perigee is from the Greek peri, about, 
and ge, the Earth, and signifies near the Earth.] 

4:54:. When is the Moon in Apogee ? 
When at her greatest distance 
from the Earth. 

[Apogee is from apo, from, and ge, the Earth.] 

455. What are the Apsides of the 
Moon's orbit ? 

They are the same as the Perigee and Apogee. 

[The singular of ap'-si-des is apsis.] 

456. What is the Line of the Apsides ? 

It is a right line from one apsis to the other. 

[This is simply the Major Axis of the elliptical orbit of the Moon. See Question 87, 
and cut.] 

457. Is the Line of the Apsides of the Moon's orbit fixed in ilie 
Ecliptic, or does it change its direction 1 

It revolves from west to east around the ecliptic in 
about nine years. 

[In the adjoining cut an attempt is made 
to represent this motion. At A the line 
of the apsides points directly to the right 
and left, but at B, C, and D it is seen 
changing its direction, till at E the change 
is very perceptible when compared with 
A. But the same ratio of change con- 
tinues ; and at the end of a year, when the 
Earth reaches A again, the line of the ap- 
sides is found to have revolved eastward 
to the dotted line I K ; or about 40°. In 
nine years the aphelion point near A will 
have made a complete revolution, and re- 
turned to its original position.] 



MOTION OF THE APSIDES. 



I 



in 



m 




• • 



""# 



PEIMAEY ASTEONOMY. 99 

458. What is the true form of the Moon's path ? 

It is an irregular curve, always concave toward the 
Sun, and crossing the Earth's orbit every 13 degrees 
nearly. 

[1. If the Earth stood still in her orbit, ^y iq0 ,^.... s 

the Moon would describe just such a path 
in the ecliptic as she describes with re- -'""'?" 3*' 

spect to the Earth. See 449, and cut. . !@& 

2. If the Earth moved but slowly on her }J.. 

way, the Moon would actually retrograde ■■ ' £* 
on the ecliptic at the time of her change, '•. ^ 
and would cross her own path at every .';-;» 
revolution, as shown in the adjoining ; g* 
figure. But as the Earth advances some **..'•,.-• 
46 millions of miles, or near 100 times the ";X[ 

diameter of the Moon's orbit, during a •..€!>..•• •. j 

single lunation, it is evident that the *'"""• "^-0- #/'•'"" 

Moon's orbit never can return into itself, "'-•—•■''*•—••'' 

or retrograde, as here represented. 

THE MOON'S ORBIT ALWAYS CONCAVE TOWARD THE SUN. 



3. That the lunar orbit is always concave toward the Sun, may be demonstrated by 
the above diagram. Let the upper curve line A B represent an arc of the Earth's 
orbit, equal to that passed through by the Earth during half a lunation. Now the 
radius and arc being known, it is found that the cord A B must pass more than 
400,000 miles within the Earth. But as the Moon departs only 240,000 from the 
Earth, as shown in the figure, it follows that she must describe the curve denoted by 
the middle line, which is concave toward the Sun. 

4. This subject may be still further illustrated by the following cut, representing 

THE MOON'S PATH DURING A COMPLETE LUNATION. 

C Tj 

P ____- -^=- — — — ^V ;•, j* . 



Here the plain line represents the Earth's orbit, and the dotted one that of the Moon. 
At A the Moon crosses the Earth's track 240,000 miles behind her. She gains on the 
Earth, till in seven days she passes her at B as a Full Moon. Continuing to gain on 
the Earth, she crosses her orbit at C, 240,000 miles ahead of her, being then at her Third 
Quarter. From this point the Earth gains upon the Moon, till seven days afterward she 
overtakes her at D as a New Moon. From D to E the Earth continues to gain, till at 
E the Moon crosses 240,000 behind the Earth, as she had done four weeks before 
at A. Thus the Moon winds her way along, first within and then without the Earth, 



100 



PRIMARY ASTRONOMY. 



always gaining upon us when outside of our orbit, and falling behind us when 
within it. 

5. The small circles in the cut represent the Moon's orbit with respect to the Earth, 
which is as regular to us as if the Earth had no revolution around the Sun.] 

459. Does the Moon ever actually retrograde upon the Ecliptic ? 
She does not. 

460. What is her absolute velocity in space, in accompanying the 
Earth around the Sun? 

It is never less than 65,700 miles per hour. 

[1. The Moon's orbitual velocity, with respect to moon's path. 

the Earth, is about 2,300 miles per hour (451). 
When outside the Earth, as at B, in the last figure, 
she gains 2,300 miles per hour, which, added to the 
Earth's velocity (215), would give 70,300 miles as 
the hourly velocity of the Moon. When within the 
Earth's orbit, as at D, she loses 2,300 miles per hour, 
which, subtracted from 68,000 miles, the Earth's 
hourly velocity, would leave 65,700 miles, as the 
slowest motion of the Moon in space, even when 
she is falling behind the Earth. 

2. Could we look down perpendicularly upon the 
ecliptic, and see the paths of the Earth and Moon, 
we should see the latter pursuing her serpentine 
course, first within and then outside our globe, somewhat as represented by the dotted 
line in the annexed figure. Her path, however, would be concave toward the Sun, as 
shown in the middle cut, on page 99, and not convex, as we were obliged to represent 
it here in so small a diagram.] 

461. How is the Moon's orbit situated with respect to the Ecliptic? 
It departs only about 5^° from that plane (5° 8' 48"). 




INCLINATION OF THE MOON S ORBIT TO THE PLANE OF THE ECLIPTIC 
C 






[Let the line A B represent the plane of the Earth's orbit, and the line joining the 
Moon at O and D, would represent the inclination of the Moon's orbit to that of the 
Earth. At C the Moon would be within the Earth's orbit, and at D exterior to it ; 
and it would be Full Moon at D, and New Moon at C] 

462. Does the Line of the Moon's nodes remain stationary on the 
Ecliptic ? 

It does not; but retrogrades or revolves westward 
around the ecliptic every 19 years. 

[The amount of this motion is 10° 35' per annum, which would require 18 years and 
219 days for a complete revolution.] 



PRIMARY ASTRONOMY. 101 



LESSON XXIY. 

THE MOON'S CHANGES. 

463. Is the Moon self-luminous, or opake 1 

She is opake, like all the rest of the planets, and 
shines only by reflection. 

[This is obvious from the fact that only so much of the Moon is bright as is enlight- 
ened by the Sun.] 

464. What is the cause of the various Phases of the Moon, as 
New Moon, Full Moon, <SfC ? 

It is caused by her revolution around the Earth, 
which enables us to see more of her enlightened side 
at one time than at another. 



CAUSE OF THE MOON S PHASES. 



2 

e 



• FULL J NEW.----'" J*" 

»CX> 0~»:3j§-4> - I 



GIBBOUS^) Q\ ®) 

LASTpR. 



ft 



[i. The above cut represents the Moon revolving eastward around the Earth. In 
the outside circle she is represented as she would appear, if viewed from a direction 
at right angles with the plane of her orbit. The side toward the Sun is enlightened in 
every case, and she appears like a half Moon at every point. 

2. The interior suite represents her as she appears when viewed from the Earth. 
At A it is New Moon, and if seen at all so near the Sun, she would appear like a dark 
globe. At B she would appear like a crescent, concave toward the east. At C more 
of her enlightened side is visible, at D still more, and at E the enlightened hemisphere 
is fully in view. We then call her a Full Moon. From E around to A again the dark 
portion becomes more and more visible, as the luminous part goes out of view, till she 
comes to her change at A. 

3. If the student will turn his book bottom upward, and hold it south of him, he will 
see why the crescent of the Old Moon at H is concave on the west, instead of the east, 

9* 



102 



PRIMARY ASTRONOMY. 



like the New Moon ; and why she is seen before sunrise instead of just after sunset. 
But these points will be called up and more fully illustrated hereafter.3 

465. What is meant by the Change of the Moon ? 

It is when she is in conjunction with the Sun, 
and chcmges from what is called an Old Moon to a 
New Moon. 

[If the student will be on the look-out, he can easily find the Moon west of the Sun 
in the day-time ; and by observing her carefully, will see that she is rapidly approach- 
ing him. In a short time she will be lost in his beams, and soon after will appear 
east of the Sun, just after sundown, as a New Moon. This change, as it is called, 
takes place when she passes the Sun eastward.] 

466. What is meant by the New Moon 1 

It is when she appears in the west like a slender 
crescent, and during the first seven days after her 
change. 

[1. It is New Moon from A to C in the preceding cut. 

NEW" MOON IN THE WEST JUST AFTER SUNDOWN. 




2. Here is a picture of what you have often seen— the New Moon in the west just 
after sundown. The Sun is scarcely out of sight, and the Moon is very close to him. 
She also will set very soon, and be out of sight. A gentleman is pointing her out to 
two boys and a little girl. They are probably some of his students going to the 
school-house near by, to a " spelling-school," or to hear a Lecture on Astronomy. 1 

467. What are the Cusps of the Moon ? 
The extremities of the crescent. 

468. What are the Moon's Syzyges ? 

Two points in her orbit 180° apart, where she is ]N"ew 
and Full Moon. (See A and E in the cut, page 101.) 



PEIMARY ASTRONOMY. 



103 



469. What are her Quadratures? 

Four points in her orbit, 90° apart. (See positions 1, 
2, 3, and 4, page 101.) 

470. What are her Octants ? 

Eight points, 45° apart. (See A, B, C, D, page 101.) 

471. What is the First Quarter 1 

It is when the Moon has performed one-quarter of 
her journey eastward around the Earth, and appears 
just one-half enlightened. 

[1. At this time the Moon is south of us as the Sun goes down, or 90° from him, 
and we see one-half of her enlightened side. 

THE MOON AT HER FIRST QUARTER. 




Here is the same gentleman we saw on the opposite page, now showing his pupils a 
Half Mootu, or the Moon at her First Quarter. A week ago she was close to the Sun as 
he went down in the west, and was only a slender crescent ; but now she is 90° east of 
the Sun, so that she is directly south, when he goes down in the west. When the 
Moon appears as here represented, she is at the point No. 2 in her orbit, as shown in 
the cut, page 101.] 

472. What is meant by the Full Moon 1 

It is when the Moon appears round, and reflects the 
greatest amount of light upon the Earth. 

473. How is the Moon, then, situated with respect to the Sun ? 
They are in opposition, or 180° apart. 

474. How is the Full Moon situated with respect 10 the Earth's 
orbit 1 



104 



PEIMAEY ASTRONOMY. 



She is outside of it. 

475. How with respect to the Sun ? 

She is at her greatest distance from him. 

[The student will understand that the Moon, by revolving around the Earth almost 
in the plane of the ecliptic, must vary in her distance from the Sun, not only as the 
Earth varies (346), but to the amount of the diameter of her own orbit, or 480,000 
miles, from New to Full Moon. See Illustration, page 101, where the Full Moon is 
shown at No. 3.] 

476. How does the Full Moon rise with respect to the Sun? 

She rises in the east just as the Sun goes down in the 
west. 

THE FULL MOON RISING AS THE SUN SETS. 




[In this picture we see the Full Moon rising in the east, as the Sun goes out of sight 
behind the hills in the west. What a splendid Moon ! She appears to rise out of the 
ocean, and to throw her silvery light upon the waves, and upon the sails of the ships 
far off at sea. The same kind teacher is out again with his students, to enjoy with 
them a walk by moonlight, and to explain to them still further the cause of the 
Moon's changes.] 

477. When is the Moon at her Third Quarter? 

When she has passed three-quarters of her journey, 
from New Moon around the Earth. (See cut, page 101.) 
47§. Is she east or west of the Sun at this time? 

West of the Sun, and going toward him. 
479. What is her distance ? 

Ninety degrees west. 

[At this time she will rise six hours before the Sun; will be south at sunrise; and 
will set at twelve o'clock.] 



PRIMARY ASTRONOMY. 



105 



480. How does the Moon appear when at her Third Quarter ? 

Just as she does at her First Quarter, except that her 
eastern side is enlightened instead of the western. 



FIRST AND THIRD 
QUARTERS. 






[At A, in the figure, we have a view of the Moon at her 
First Quarter, when she is south as the Sun sets, and her 
western limb is enlightened. At B we see her as she ap- 
peal's when at her Third Quarter, when she is on the 
meridian as the Sun rises, and her eastern limb is en- 
lightened.] 

4§1. What is meant by the Old Moon? 
She is called an Old Moon from the Full to the New 
and especially from the Third Quarter to the change. 

482. How does the Old Moon appear? 

Like a slender crescent, much like the ISTew Moon. 

483. Wherein do they differ ? 

The cusps of the New Moon point east, and those of 
the Old Moon west. 

484. When and where can we best see the Old Moon? 
In the east, just before sunrise. 

THE OLD MOOX BEFORE SUNRISE. 




[1. Here the Old Moon is seen in the east just before sunrise. It looks just like the 
New Moon shown on page 102, except that the crescent is inverted, the concave side 
being west instead of east. The teacher is pointing to the Moon, and explaining the 
difference between her present appearance and that of the New Moon, and the cause 
of that difference. His pupils have become so interested in the subject as to be up and 
dressed before, sunrise, to see the Old Moon. 

2. The Moon is at this time very near her change. At noon she may be seen just 



106 PEIMAEY ASTKONOMY. 

west of the Sun, and in a few days will pass him eastward, when she will be a New 
Moon again, seen in the west as the Sun goes down, as represented on page 102. 
Thus she continues to pass through her changes every 29| days from age to age. 

3. We earnestly recommend to both teacher and student to observe the present 
place and appearance of the Moon, and watch her through one lunation at least. 
A little time spent in this way will do more to fix correct ideas in the mind than months 
of abstract study.] 

485. When is the Moon Gibbous ? 

When between a Half and a Full Moon. (See " Gib- 
bons," cut, page 101.) 

486. What is meant by the Moon's Waxing and Waning ? 
She waxes larger from the change to the full ; and 

wanes, or grows smaller, from the full to the change 
again. 



LESSON XXY. 

DAY AND NIGHT, SEASONS, AND TELESCOPIC APPEARANCE OF 
THE MOON. 

487. What is the line called which separates the dark from the en- 
lightened portion of the Moon's disk ? 

The Terminator. 

[As just one-half of the Moon is always enlightened by the Sun, whether it appears 
so to us or not, it follows that the Terminator must extend quite around the Moon, 
dividing the enlightened from the unenlightened hemisphere. This circle is called 
the Circle of Illumination. At New and Full Moon this circle is sidewise to us, but at 
the First and Third Quarters it is edgewise. The portion of the Terminator visible 
from the Earth traverses the Moon's disk twice during every lunation.] 

488. Has the Moon a Diurnal Revolution ? 
She has. 

489. In what time does she revolve on her Axis ? 

In 29^- days, or once during every revolution around 
the Earth. 

490. How is this known 1 

From the fact that the same side of the Moon is 
always toward the Earth. 



PKEtfARY ASTROXOMY. 



107 



MOON S REVOLUTION. 






c 



JO 



A -l) 



X) 



[1. By watching the Moon carefully with the 
naked eye, it will be seen that the same spots 
occupy nearly the same places upon her disk 
from month to month ; which shows that the 
same side is always toward us. 

2. Suppose a monument erected upon the 
Moon's surface, so as to point toward the Earth 
at A'ew Mooru, as represented at A. From the 
Earth it would appear in the Moon's center. 
Now if the Moon so revolved upon her axis, in 
the direction of the arrows, as to keep the pillar 
pointing directly toward the Earth, as shown at 
A, B, C, and D, and the intermediate points, she 
must make just one revolution on her aris during ^/ 

her periodic revolution. At A the pillar point3 

from the Sun, and at C toward him ; showing that, in going half way round the 
Earth, she has performed half a revolution upon her axis.] 

491. Wliat is meant by the Moon's Lib rations 'i 

A slight apparent rolling motion, first one way and 
then the other. 

492. How are her Librations distinguished ? 

As librations in Latitude, and librations in Longitude. 

493. What are her Librations in Longitude ? 

A motion by which more of her eastern and western, 
borders alternately appear and disappear. 

494. What is the cause of this Librationl 

It is because her angular motion in her elliptical 
orbit is not uniform, like her motion around her axis. 

[1. From A around to C the angular motion is 
slower than the average, and the diurnal motion 
gains upon it, so that the pillar points west of the 
Earth, and we see more of the eastern limb of 
the Moon. 

2. From C to A, again, the Moon advances m j 
faster than a mean rate, and gains upon the 
diurnal revolution ; so that the pillar points east 
of the Earth, and we see more of the Moon's 
western limb. Thus she seems to Iibrate or roll, •^ 
first ODe way and then the other, during every A J-C 
periodic revolution. 

3. At B we see most of her eastern limb, and 
at D most of her western.] g 



MOON'S LIBRATIONS. 



Q 



*> 



\ 



«€ 



495. What 
Latitude ? 



is her Libration 



6 



ft> 



108 PRIMAKY ASTBONOMY. 

A slight rolling motion, by which the parts about her 
poles alternately appear and disappear. 

496. What is the cause of this ? 

The inclination of her axis to the plane of her orbit, 
and her revolution around the Earth. 

497. What is the inclination of her Axis to her Orbit? 
About a degree and a half (1° 30' 10 ;A .8). 

[If the inclination of the Earths axis brings first one pole and then the other tow ard 
the Sun, and produces the Seasons (347), so the inclination of the Moon's axis would 
bring first one pole and then the other in view from the Earth. But as- her inclination 
is only 1£°, the libration in latitude is very slight.] 

49§. What is the Length of a Natural Day upon the Moon 1 
Twenty-nine and a half of our days. 

[A complete revolution upon her axis gives her a Natural Day • aisi if ih&% requires 
294 of our days, her day must be equal to 29£ of ours.] 

499. What is the Length of her Year ? 
Only 29J days. 

[By the year of a planet, we mean the time required for a complete revolution in its 
orbit, during which it must pass through all its seasons. Hence we speak of the year 
of Herschel as being equal to 84 of our years ; and that of Neptune as equal to 164. 
(See Question 212.) By applying this rule to the Moon, and measuring her year by 
her periodic revolution, we find it compressed within the narrow limits of 29| days.] 

500. What curious fact does this establish ? 

That she has only one day in her year ; or, in other 
words, that her days and her years are precisely of a 
length. 

501. Can the Earth be seen from all parts of the Moon ? 

She can only be seen from the one side, which is 
always turned toward us. 

502. How would the Earth appear from the Moon ? 

Like a bright stationary planet, thirteen times larger 
than the Moon, and exhibiting all her varying phases. 

[Turn back and consult the figure, page 101. To the lunarian at A it would be 
night, and the Earth would appear like a magnificent Full Moon. At B she would be 
at her Third Quarter, and at C like the Moon at her changes, &c. But whatever might 
be the Earth's phase or appearance, she would always appear stationary, or occupying 
the same position in the heavens. From the apparent center of the Moon the Earth 



PEIMAEY ASTEONOMY. 



109 



NATURAL APPEARANCE OF THE 
FULL MOON. 



would appear directly overhead, while 90° from that point she would always appear 
in the horizon.] 

503. How does the surface of the Moon appear to the naked 
eye? 

It exhibits a variety of dark lines and spots. 

504. What do they resemble ? 

There is a dark figure on her western limb, resem- 
bling that of a man, with his head to the north, and 
his body inclined to the east. Just east of him, and 
opposite his shoulders, is an irregular object resembling 
a huge hundle or pack. 

[1. Both these objects are represented in the ad- 
joining cut, which was drawn from nature by the 
author on the evening of December 18, 1850. It 
represents the Moon as she appears when about 
two hours high, and is the best of six different 
sketches taken during the same evening. Let the 
student compare it with the next Full Moon, and 
see if our drawing is correct. 

2. The Ojibway Indians have a legend by 
which they explain this singular appearance of 
the Moon. Instead of a "man," they say this 
figure is a beautiful Ojibway maiden, who was 
translated to the Moon "many snows ago," for 
having set her affections upon that object, and 
refusing to marry any of the " young braves" of 

the Ojibway nation. How the " beautiful maiden" came to look so coarse and mas- 
culine, and what the rest of the figure means, the tradition does not inform us. 

3. In answer to inquiries sometimes made by children as to this " man," they have 
been told that it is a man with a bundle of wood upon his back, who was sent to the 
Moon for his wickedness, in gathering sticks on the Sabbath. This story is less inno- 
cent than the Ojibway tale, as it trifles with the subject of disobedience to God, and 
with the sanctity of the holy Sabbath.] 

505. What are these objects supposed to be ? 

The outlines of her great natural divisions, as Mount- 
ains, Yalleys, and Continents. 

[Almost every body has noticed these rude outlines upon the face of the Moon, and 
many, doubtless, have wondered what they w r ere ; but how few have supposed, as they 
were gazing upon her mottled disk, that they were enjoying a distant view of a world; 
and that these dim outlines were a natural map of its nearest hemisphere ! Having 
seen " the Man in the Moon," they have supposed it useless to pursue the subject any 
further, and here their investigations have ended.] 

506. Hoio does the Moon appear through a Telescope 1 



10 




110 



PRIMARY ASTRONOMY. 



Her surface is very rough and uneven, covered with 
deep valleys and lofty mountains. 

[The profile of the Moon is remarkably characterized by inequalities like our mount- 
ain ranges. This, indeed, is its most striking features. There are a great number of 
perfectly isolated conical peaks, or sugar-loaf mountains, springing out of the plains, 
and also several magnificent chains or ridges, some of whose peaks are 25,000 feet high. 
The chain called the Apennines, represented in the following cut, is not surpassed by 
any of the ranges of our globe. They are most precipitous on the side toward the 
plain country, and gradually slope off through thousands of minor peaks on the oppo- 
site declivity ; thus conforming to what seems to be a law among our terrestrial 
ranges ; viz. they are steep, almost precipitous, on one side, and their other is a long 
slope. See Nichol on the Solar System.] 

507. What proof have we that the Moon's surface is mountainous 1 

TVlA llTlP A"P tllP TELESCOPIC APPEARANCE OF THE MOON. 

terminator is very 
rough and uneven, 
which would not 
be the case if her 
surface was level or 
smooth. 

[See line dividing the il- 
luminated from the dark por- 
tion in the annexed cut.] 

50 §. Have we any 
other proof? 

The projection of 
long shadows, in a 
direction opposite the Sun, shows the existence of 
mountains that intercept the Sun's light. 

[These shadows may be seen in the above cut, projected from right to left.] 
509, Can you mention any further evidence of the existence of 
Mountains in the Moon ? 

From New to Full Moon bright spots break out from 
time to time, just east of the terminator, in the dark 
portion, and grow larger and larger, till they join the 
illuminated portion, showing them to be the tops of 
mountains, which reflect the sunlight before it reaches 
the intervening valleys. 




PRIMARY ASTRONOMY. 



Ill 



[Specimens of these bright points may be seen in the cut. The writer has often 
watched them, and seen them enlarge more and more, as the Sun arose and enlightened 
the sides of the mountains.] 

510. When is the best time to examine the Moon with a Tele- 
scope ? 

Near the First or Third Quarter. 

511. Why is this? 

Because the shadows of objects are then seen at right 
angles with the line of vision, and to the best advan- 
tage ; while at Full Moon objects cast no shadows vis- 
ible to us. 

512. How are the Shadows projected from the Full to the New 
Moon? 

From east to west. 



OLD MOON. 



FULL MOON. 



NEW MOON. 




[1. The shadows are always projected in a direction opposite the Sun, or toward the 
dark side of the Moon ; and as her eastern limb is dark from the change to the full, 
and her western from the full to the change, of course the direction of the shadows 
must be reversed. 

2. Suppose a person stationed at a distance directly over the Andes. Before the 
Sun arose, he would see the tallest peaks enlightened, and as he arose the long 
shadows of the mountains would extend to the west. At noon, however, little or no 
shadow would be visible ; but at sunset they would again be seen stretching away to 
the east. This is precisely the change that is seen to take place with the lunar shad- 
ows, except that the time required is a lunar day, equal to about 15 of our days, instead 
of a terrestrial day of 12 hours.] 

513. What is the Form of the Lunar Mountains ? 

Some of them are in extensive ranges, while others 

are of a circular form. 

[Great numbers of these circular mountains may be seen with a telescope of 
moderate power. Through such an instrument the Moon will appear of a yellowish 



112 PEIMAEY ASTRONOMtf. 

hue, and the circular mountains like drops of thick oil on the surface of water. Two 
extensive ranges and several of the circular elevations are shoWn in the cut, 
page 110.] 

514. What is their estimated Hight ? 
From three to five miles. 

[Almost every common Arithmetic has rules for determining the hight of objects 
by the length of their shadows ; and by applying these rules to the shadows seen upon 
the Moon's surface, astronomers ascertain the hight of the lunar mountains.] 

515. What is inferred from the Shape of the Circular Mount- 
ains ? 

That they are craters of immense volcanoes. 

516. Is the Moon surrounded by an Atmosphere % 

It is not certain whether she is or not : if she is, it 
must be exceedingly thin or rare. 

[The substance we call air or atmosphere is subject to the general law of gravitation. 
Hence it is most dense at the Earth's sm-face, and grows rare as we ascend. Inas- 
much, therefore, as the general density of the atmosphere of any planet is dependent 
upon the attracting force of that planet; and the Moon has only about 73d part as 
much attracting power as the Earth ; it follows that her atmosphere, if she has one, 
ought to be much less dense than ours.] 

SIT. Has she any Water upon her surface 1 

It is thought not, from the fact that it would be con- 
verted into steam or vapor, during her long and hot 
days, and from the fact that no clouds are ever seen 
floating around her. 

518. Was it ever supposed that she had Seas upon her sur- 
face ? 

It was ; but the portions once supposed to be seas, are 
now found to be only prairies or plains. 

519. How does the Moon appear through Lord Ross's great Tele- 
scope ? 

Dr. Scoresby, who examined her through this instru- 
ment, states that she appears " like one vast ruin of 
nature," with numerous volcanoes, and fragments of 
rock scattered about them in every direction. 

520. What does Dr. Herschel say of the Lunar Volcanoes 1 



PEIMART ASTEONOMY. 



113 



He believed that lie actually saw the Jvres of several 
that were in active eruption. 

[This has not been confirmed by any recent observer, and is therefore somewhat 
doubtful.] 



LESSON XXYI 



SHADOWS OF THE PLANETS. 



ECLIPSES OF THE SUN. 

521. What is an Eclipse ? 

It is a partial or total obscuration or darkening of the 
Sun or Moon, by the intervention of some opake body. 

522. How many kinds of Eclipses are there ? 

The first general division is into Solar and Lunar. 

523. What is a Solar Eclipse? 
An Eclipse of the Sun. 

524. What is a Lunar Eclipse ? 

An Eclipse of the Moon. 

525. What is the first 
point to be observed in consid- 
ering the subject of Eclipses ? 

The fact that all the 
planets, both primary 
and secondary, cast 
shadows in a direction 
opposite the Sun. 

526. Upon what does the 
form and length of these 
shadows depend? 

Upon the comjpara- 
t/we magnitude of the 
Sun and planet, and 
their distance from each other. 



\ 



\ 



10* 



114 



PRIMAET ASTEONOMY. 



527. What would be the form of the Shadow if the Sun and 
planet were of a size ? 

The shadow would be in the form of a cylinder. 



CYLINDRICAL SHADOW. 







528. What would be the form of the Shadow if a planet were 
larger than the Sun ? 

It would diverge or expand from the planet outward. 



DIVERGING SHADOW. 




H 5 



529. As the planets are much smaller than the Sun, what must 
be the form of their Shadows ? 

They must converge to a point, taking the shape of 
a cone. 



CONVERGING SHADOW. 






i( 



[By observing the cut on the preceding page, the student will see the shadows of 
the planets all running to a point, in accordance with this principle.] 

530. What effect has the distance of a planet upon the form and 
length of its Shadow? 

The more distant, the longer its shadow, and the 
more slender the point of the cone. 

SHADOW MODIFIED BY DISTANCE. 






[1. In this cut, the Sun and Earth are of the same size as in the one immediately 



PRIMARY ASTRONOMY. 



115 



preceding, and yet this shadow is shorter and the cone more abrupt than in the other 
case, simply because the two bodies are here placed nearer each other. 

2. By turning again to Question 526, and the cut, you will see that the bodies near the 
Sun have comparatively short shadows, and the cones terminate quite abruptly 
while those more distant have longer and more slender shadows. No primary, how- 
ever, casts a shadow long enough to reach the next exterior planet.] 

531. What is the cause of a Solar Eclipse ? 

It is caused by the Moon passing between the Earth 
and the Sun, and casting her shadow upon the Earth. 

532. At what time of the Moon must they always occur? 

At New Moon. 

533. Why is this? 

Because she is never between the Earth and the Sun 
except at the time of her change. 

534. Why do we not have a Solar Eclipse at every New Moon ? 
Because the plane of the Moon's orbit is not in the 

plane of the ecliptic ; so that she sometimes passes 
above the Sun, and sometimes below him. 

NEW MOONS WITHOUT ECLIPSES. 
Abovi the Sun. 







Below the Sun. 



[Let the line joining the Earth and the Sun represent the plane of the ecliptic. Now 
as the orbit of the Moon departs from this plane 5° 9', as shown and illustrated at 461, 
she may appear either above or below the Sun at New Moon, as represented in the 
figure ; and her shadow fall above the North Pole or below the South. At such times, 
then, there can be no eclipse.] 

SOLAR ECLIPTIC LIMITS. 




116 



PRIMARY ASTRONOMY. 



SOLAR ECLIPSE. 



535. Why do Eclipses of the Sun always come 
on from the West and pass over Eastward 1 

Because the Moon, which causes them, 
revolves from west to east. 




[1. In the adjoining cut, the Moon is seen revolving eastward, 
throwing her shadow upon the Earth, and hiding the western 
lirnb of the Sun. In some instances, however, when the eclipse 
is very slight, it may first appeal- on the northern or southern 
limb of the Sun ; that is, the upper or lower side ; but even then 
its direction must be from west to east. 

2. It will also be obvious from this figure, that the shadow of 
the Moon upon the Earth must also traverse her surface from 
west to east. Consequently, the eclipse will be visible earlier in 
the west than in the east.] 

536. Where must the Moon be in her orbit at 
the time of her change, in order to eclipse the Sun ? 

At or near one of her Nodes. 

537. What is the greatest distance at which she 
may be from either Node, and yet eclipse the Sun ? 

About seventeen degrees (16° 59'). 

538. What is this distance called ? 

The Solar Ecliptic Limits. 

[1. This point is illustrated by the last cut on the preceding 
page. At A the line of the Moon's nodes points directly toward 
the Sun, so that in passing her ascending node at B, the Moon passes centrally 
between him and the Earth, and produces a total eclipse. At C, also, the Moon 
would pass the Earth's shadow centrally, and would be totally eclipsed. 

2. At D the case is different. The line of the Moon's nodes points east of the Sun, 
and she reaches her conjunction at H when seventeen degrees from her descending 
node at F. There is now no eclipse, as the Moon is too far from her node, and conse- 
quently too high above the ecliptic. The distance from F to H represents her 
Ecliptic Limits, within which she must be at her change in order to eclipse the Sun. 

THE MOON CHANGING AT DIFFERENT DISTANCES FROM HER NODES. 





3. Let the line A B represent the plane of the Earth's orbit, and C D that of the 
Moon. Now if the change occurred when the Moon was exactly at her node, and the 
Earth on the line of her nodes (as shown at E, where the Moon is supposed to be 
beyond the Earth, and out of sight), the Moon would be in the plane of the ecliptic, 
and would appear to pass directly over the Sun's center. Such an eclipse would, 
therefore, be central, and either total or annular. 



PEIMAET ASTEONOMY. 117 

4. If the change took place when the Moon was either side of her node as at F F, 
her center would be either above or below the ecliptic, and she would appear to cros3 
the upper or lower part of the Sun. The eclipse, therefore, would be partial. 

5. At G G the eclipse would be still less ; at H H the two disks (Sun and Moon) 
would seem just to touch each other ; and at K K and all points more distant from the 
nodes there would be no eclipse whatever, as the Moon would seem to pass entirely 
above or below the Sun. The points H H represent the Solar Ecliptic Limits.] 

539. When, therefore, may we expect Eclipses of the Sun ? 
Whenever the Moon is within 17° of either Node at 

the time of her change. 

540. What is meant by the Umbra of the Earth and Moon ? 

It is the dark shadow cast in a direction opposite the 
Sun. 

[ Umbra is a Latin word, signifying a shade or shadow."] 

541. What is the Penumbra ? 

It is Si partial shadow outside the Umbra. 

\_Pe-num-bra is from the Latin pene, almost, and umbra, a shadow ; and signifies 
almost a shadow.'] 

UMBRA AND PENUMBRA OF THE EARTH AND MOON. 




[1. In this cut, the Earth's umbra and penumbra will be readily found by the let- 
tering, while A is the umbra and B B the penumbra of the Moon. The latter is more 
broad than it should be, owing to the nearness of the Sun in the cut, as it never 
extends to much over half the Earth's diameter. 

2. The student will see at once that solar eclipses can be total only to persons 
within the umbra; while to all on which the penumbra falls a portion of the Sun's 
disk will be obscured.] 

542. What is the average length of the Moon's Umbra ? 
About 239,000 miles. 

[It varies from 221,148 to 252,638 miles, according to the Moon's distance from tho 
Sun. See 526, and cut.] 

543. What is its greatest diameter at the distance of the Earth ? 
About 170 miles. 

[This is when it strikes the Earth centrally or perpendicularly to its surface. When 
it strikes obliquely, it covers a much larger surface.] 



118 PEIMAEY ASTKONOMY. 

544. How large a portion of the Earth's surface may be covered 
by the Moon's Penumbra? 

About 4,393 miles in diameter. 

545. What is a Partial Eclipse ? 

One in which only part of the Sun or Moon is ob- 
scured. 

546. When is an Eclipse total? 

When the whole disk of the Sun or Moon is dark- 
ened. 

54T. What is an Appulse? 

It is when the Sun and Moon, or the Moon and the 
Earth's shadow, seem just to touch, without producing 
an actual eclipse. (See H H in the cut, page 116.) 

54§. At what places on the Earth are Solar Eclipses generally 

TOTAL? 

At all places within the Moon's Umbra. 

549. Where are they partial ? 

At all places beyond the Umbra and within the 
Penumbra. 

550. What is meant by a Central Eclipse of ike Sun ? 

It is when the Moon changes when exactly at one of 
her Nodes, and appears to pass centrally over the Sun's 
disk. 

551. Are all Central Eclipses of the Sun necessarily total? 
They are not. 

552. If they are central, but not total, how must the Sun appear ? 
Totally obscured, with the exception of a bright ring, 

apparently around a dark body in the center. 
558. What are such Eclipses called? 
Annular Eclipses. 
554. Why are they called Annular ? 
From annulus, a ring, because the Moon only hides 



PKIMARY ASTRONOMY. 119 

the center of the Sun, and leaves a bright ring unob- 
scured. 

PROGRESS OF A CENTRAL ECLIPSE. 

Going off. Annular. Coming on. 




555. When will the next Annular Eclipse visible in the United 
States occur? 

May 26, 1854. 

556. Why are some Central Eclipses of the Swi Total and 
others Annular? 

Because the apparent magnitude of the Sun and 
Moon varies as their distances vary. (See 178, and Il- 
lustrations.) 

TOTAL AND ANNULAR ECLIPSES. 
Total Annular. 

» 




[1. At A the Earth is at her aphelion, and the Sun, being at his most distant point, will 
have his least apparent magnitude. At the same time the Moon is in perigee, and 
appears larger than usual. If, therefore, she pass centrally over the Sun's disk, the 
eclipse -will be total. 

2. At B this order is reversed. The Earth is at her perihelion, and the Moon in 
apogee; so that the Sun appears larger and the Moon smaller than usual. If, then, a 
central eclipse occur under these circumstances, the Moon will not be large enough to 
eclipse the whole of the Sun, but will leave a ring, apparently around herself, unob- 
scured. Such eclipse will be annular.'] 

557. What are the effects of a Total Eclipse of the Sun ? 
The heavens are shrouded in darkness, so that stars 
and planets are visible; the temperature declines ; the 



120 



PRIMARY ASTRONOMY. 



animal tribes become agitated; and a general gloom 
overspreads the landscape. 

[Such were the effects of the great eclipse of June, 1806.] 

558. When will the next Total Eclipse of the Sun, visible in the 
United States, occur ? 

August 7th, 1869. 



LESSON XXYII. 



ECLIPSES OP THE MOON. 



LUNAR ECLIPSE. 



559. What is the cause of Lunar Eclipses ? 

The Moon's passing through a portion of the Earth's 
shadow. 

560. In what time of the Moon do they always 
occur ? 

At Full Moon. 

561. Why is this? 
Because the Moon is always full when 

in opposition to the Sun ; and the Earth's 
shadow being in opposition, the Moon 
must pass through it when full, if at all. 

562. Why do all Eclipses of the Moon begin 
on the East, and pass over Westward, contrary to 
the direction of a Solar Eclipse? 

Because, as the Moon revolves east- 
ward, her eastern limb first comes in con- 
tact with the Earth's shadow. 




[By holding the book up south of him, the student will see at 
once why the eastern limb of the Moon must be first eclipsed, 
and why the shadow seems to pass over westward.] 

563. Why have we not a Lunar Eclipse at every 
Full Moon ? 



(fftok 



4JJ 



PRIMAEY ASTEONOMT. 121 

For the same reason that we have not a Solar Eclipse 
at every New Moon ; namely, because the Moon's orbit 
is not in the plane of the ecliptic, where the Earth's 
shadow lies. 

NEW AND FULL MOONS WITHOUT ECLIPSES. 

Shadow above the Earth. Above the Earth's shadow. 




Shadow below the Earth; Below the Earth's shadow 



[Here it will be seen that as the Moon's orbit departs 5° 9' from the plane of the 
ecliptic, she may pass either above or below the Earth's shadow, and therefore not 
be eclipsed by it.] 

564. Where must the Moon be in her orbit, at the time of her op- 
position, in order to be eclipsed? 

At or near one of her Nodes. 

565. What are her Ecliptic Limits ? 
About twelve degrees (11° 25' 40"). 

566. What is the average Length of the Earth's Shadow 1 
About 860,000 miles. (See cut, page 116.) 

[Its length varies from 842,217 to 871,262 miles, according to the Earth's distance 
from the Sun. See 113, and cut.] 

567. What is its average Breadth at the distance of the Moon ? 
About 6,000 miles. 

[This breadth also varies from 5,232 to 6,365 miles.] 

568. How long, then, may an Eclipse of tlie Moon last 1 
If central, it may last four Twurs. 

569. What is the natural progress of a Lunar Eclipse ? 

As the Moon enters the Earth's Penumbra, she loses 
a portion of the Sun's light, and begins to grow pale or 
dusky, till at length she enters the Umbra, and is 
really eclipsed. 

5YO. Can an Eclipse of the Moon ever be Annular ? 

11 



122 



PEIMAEY ASTKONOMY. 



It cannot. 

571. Why not? 

Because the diameter of the Earth's shadow, where 
the Moon passes it, is always greater than the diameter 
of the Moon. 

572. What is the greatest and least number of Eclipses that can 
ever occur in one year ? 

There can never be less than two, nor more than 

seven. 

573. What is the most common number? 
Four. 

574. How do astronomers record the extent of Solar and Lunar 
Eclipses ? 

By dividing the diameters of the Sun and Moon into 
twelve equal parts, called Digits, and observing how 
many of these parts are eclipsed. 



FIVE DIGITS ECLIPSED. 




TWELVE DIGITS. 

'A\\ri||i>., 




'"^-'i,,*,.* 



575. How were Eclipses regarded by the Ancients ? 

With amazement and fear ; as supernatural events, 
indicating the displeasure of the gods. 

576. What use did Columbus make of this Superstition ? 
When the inhabitants of St. Domingo refused to 

allow him to anchor, in 1502, or to furnish him sup- 
plies, he told them the Great Spirit was offended at 
their conduct, and was about to punish them. In 
proof, he said the Moon would be darkened that very 



PEEMARY ASTKONOMY. 123 



night, for he knew an eclipse was to occur. The arti- 
fice led to a speedy supply of his wants. 



LESSON XXVIII. 

SATELLITES OF JUPITER, ECLIPSES, ETC. 

577. How many Moons has Jupiter ? 
Four. 

578. Are tJiey easily seen ? 

They are with a spy-glass or telescope, but not with 
the naked eye. (See note after 405.) 

[It is said, however, that one or two of them may occasionally be seen with the 
naked eye ; but such occasions and such eyes will rarely be met with.] 

579. Who first discovered them ? 
Galileo, the inventor of the telescope. 

580. How are they distinguished ? 

As first, second, third, and fourth, according to their 
distances from their primary. 

581. What is their Size or Magnitude ? 

They are all a little larger than our Moon, except the 
second, which is a trifle less. 

582. How are they situated as to distance from Jupiter ? 

The first is about the distance of our Moon, and the 
others respectively about two, three, and five times as 
far off. 

COMPARATIVE DISTANCES OF JUPITER'S MOONS. 
4th. 3d. 2d. 1st. 




583. In what time do they revolve about their Primary ? 
From 1 day 18 hours to IT days ; according to their 
respective distances. 



124 



PEIMAEY ASTRONOMY. 



[Though the fourth satellite is 1,164,000 miles from Jupiter, or about five times the 
distance of our Moon, she revolves around her primary in seventeen days !] 

584. Why do the Moons of Jupiter revolve so rapidly ? 

In order to counterbalance his powerful centripetal 
force or attraction, and keep the satellites from falling 
to his surface. 

585. How are the Orbits of these Moons situated ? 

They are all in or near the plane of his equator. 
(See representation in the last cut). 

586. How would this place them with respect to the Ecliptic ? 
As Jupiter's orbit and axis are but slightly inclined, 

his equator nearly coincides with the ecliptic ; and if 
his satellites revolve near the plane of his equator, they 
must also be near the ecliptic. 

587. How do these Satellites 



TELESCOPIC VIEWS OF THE MOONS 
OF JUPITER. 



usually appear to be situated ? 

At different distances ; 
some on one side of the 
primary, and some on the 
other. 

588. What is their apparent 
motion 1 

They seem to oscillate 
like a pendulum, from their 
greatest elongation on one 
side, to their greatest elon- 
gation on the other. 

[If we could look down perpendicularly upon the ecliptic, we should see these satel- 
lites revolving in apparent circles ; but as wo are in or near the plane of the ecliptic, 
which is the plane of their orbit, they seem merely to pass to and from the plane.] 

589. What is the Form of the Orbits of these Satellites ? 
They are very nearly circular. 

[This fact is ascertained by observing that their greatest elongalion is nearly the 
i same both east and west at every revolution; whereas, if their orbits were very ellip- 
I tical, their greatest elongations would vary. See Question 247, and Notes.] 




PEEMAEY ASTEONOMY. 125 

590. In what direction do they revolve ? 

From east to west, or in the direction their primary 
revolves, both upon his axis and in his orbit. 

[See the directions as indicated by the arrows in the next cut.] 

591. Have they a revolution around their respective Axes ? 

They are supposed to revolve once upon their axes 
during every revolution around their primary, as is the 
case with our own satellite. 

592. Are Jupiter s Moons always visible ? 

They are not. Sometimes only one or two can be 
seen. (See the lower figure in the opposite cut.) 

593. Why is this ? 

Because, as their orbits lie near the plane of Jupiter's 
orbit, they have to pass his broad shadow, and be 
totally eclipsed at every revolution. 

eclipses of jupiter's moons. 
.--•" " v*D \ r . 



KC CO! OA 




B 



[1. In this cut we have a perpendicular view of the orbits of Jupiter's satellites, and 
they appear like circles. The first Moon is in an eclipse. 

2. To a person on the Earth at A, the fourth Moon would seem to pass and repass 
from B to C ; and so with the other three, according to their respective distances.] 

594. Are there no exceptions to the total eclipse of these Satellites 
at every revolution 7 

There is one exception. As the fourth satellite de- 
parts about 3° from the plane of Jupiter's orbit, and is 
quite distant, it sometimes passes above or below the 
shadow, and escapes eclipse. But such escapes are not 
frequent. 

_ 



126 PRIMARY ASTRONOMY. 

595. Do these Satellites ever eclipse Jupiter ? 

Their shadows are often thrown upon his bright 
disk, and may be seen like small round ink-spots, j 
traversing it from side to side. 

[The second satellite is thus eclipsing Jupiter in the preceding cut.] 

596. What kind of an Eclipse is it upon Jupiter, when the shadow 
of a Satellite falls upon the primary ? 

An Eclipse of the Sun, or Solar Eclipse. 

597. How many Eclipses, visible from Jupiter, take place every 
month ? 

About forty. 

59 §. When a Satellite goes into Jupiter's shadow, what is it 
called? 

Its Immersion / because it is immersed, hid, or 
buried in the shadow. 

599. What is its coming out of the shadow called 1 

Its Emersion, because it emerges or comes out. 

600. Can these Immersions and Emersions always be seen ? 
They cannot; as the position of the Earth in her 

orbit is sometimes unfavorable for such observations. 



% lOt @E 



^y 



[If the Earth were at A in the cut, the immersion, represented at C, would be in- 
visible ; and if at B, the emersion at D could not be seen. So, also, if the Earth were 
at F, neither could be seen ; as Jupiter and all his attendants would be directly beyond 
the Sun, and would be hid from view.] 

601. How may the system of Jupiter and his Satellites be re- 
garded ? 

As a miniature representation of the Solar System ; 



PEIMAKY ASTRONOMY. 127 



and as furnishing triumphant evidence of the truth of 
the Copernican theory. 

602. In what other light may it be viewed? 

As a great Natural Clock, keeping absolute time 
for the whole world. 

603. How is time marked by this system ? 

By the immersions and emersions of the satellites. 

604. What use can we make of the time thus denoted ? 

We may ascertain the longitude of any place upon 
the Earth's surface. 

[1. By long and careful observations upon these satellites, astronomers have been 
able to construct tables, showing the exact time when each immersion and emersion 
will take place, at Greenwich Observatory, near London. 

2. Suppose the tables fixed the time for a certain satellite to be eclipsed at 12 
o'clock at Greenwich, but we find it to occur at 9 o'clock, for instance, by our time. 
This would show that our time was three hours behind the time at Greenwich ; or 
in other words, that we were three hours, or 45° west of Greenwich. If our time was 
ahead of Greenwich time, it would show that we were east of that meridian, to the 
amount of 15° for every hour of variation. See Question 270 to 274, page 59.] 

005. What great discovery was made by observations upon the 
Eclipses of Jupiter's Moons ? 

The progressive motion and velocity of light. 

[This discovery may be illustrated by again referring to the opposite cut. In the 
year 1675 it was observed by Roemer, a Danish astronomer, that when the Earth was 
nearest to Jupiter, as at E, the eclipses of his satellites took place 8 minutes 13 
seconds sooner than the mean time of the tables ; but when the Earth was farthest 
from Jupiter as at F, the eclipses took place 8 minutes and 13 seconds later than the 
tables predicted: the entire difference being 16 minutes and 26 seconds. This differ- 
ence of time he ascribed to the progressive motion of light, which he concluded re- 
quired 16 minutes and 26 seconds to cross the Earth's orbit from E to F.] 

606. What is the estimated Velocity of Light ? 

About 200,000 miles per second (192,697). 

[16 m. 26 s. = 986 s. If the radius of the Earth's orbit be 95 millions of miles, the 
diameter must be twice that, or 190 millions. Divide 190,000,000 miles by 966 
seconds, and we have 192,697^ ^f miles as the progress of light in each second. At 
this rate, light would pass nearly eight times around the globe at every tick of the 
clock ; or nearly 500 times eveiy minute !] 

60Y. Is there any thing more rapid than Light ? 

Electricity is supposed to move about one -third 
faster ; or near 300,000,000 miles per second. 



128 PEIMAET ASTRONOMY. 



LESSON XXIX. 

SATELLITES OF SATURN, HERSCHEL, AND NEPTUNE. 

608. How many Moons has Saturn ? 

Eight. (See 420, and cut, page 9.) 

609. Are they visible to the naked eye ? 

They are not ; and can only be seen with telescopes 
of considerable power. 

610. When is the best time to observe them ? 

When Saturn is at his equinoxes, and his rings nearly 
invisible. 

[In January, 1849, the author saw satellites of saturn. 

five of these satellites, as repre- 
sented in the adjoining cut. The 
rings appeared only as a line of 
light, extending each way from the 
planet, and the satellites were in 
the direction of the line, at different 
distances, as here represented. See Note, after 420.] 

611. What is the Form and Position of their Orbits? 

They are nearly circular; and all except the eighth 
revolve in the plane of the rings. (See the above cut.) 

612. In what direction do they revolve ? 

From west to east, or with the planet and his rings. 

613. What are their Distances ? 
From 123,000 to 2,366,000 miles. 




(0> 



COMPARATIVE DISTANCE OF THE MOONS OF SATURN. 
13 3 4 b 6 7 



614. What are their Periods of Revolution! 

From 22 hours to 79 days, according to their re- 
spective distances. 

615. Is any thing known of the Magnitude of Saturn's Moons ? 



PRIMARY ASTRONOMY. 129 

The most distant is the largest, supposed to be about 
the size of Mars ; and the remainder seem to grow 
smaller, according to their respective distances. 

616. Do they suffer any Eclipses ? 

Very seldom, except when the rings are seen edge- 
wise. 

[1. Let the line A B represent the n i ■ 

plane of the planet's orbit, C D his 

axis, and E F the plane of his rings. "'- ' 

The satellites being in the plane of the 
rings, will revolve around the shadow 
of the primary, instead of passing 
through it and being eclipsed. 

2. At the time of his equinoxes, 
however, when the rings are turned toward the Sun (see A and E, cuts, page 91), they 
must be in the center of the shadow on the opposite side ; and the Moons, revolving 
in the plane of the rings, must pass through the shadow at every revolution. The 
eighth, however, may sometimes escape, on account of his departure from the plane of 
the rings, as shown in the cut.] 

617. Do these Satellites revolve upon their respective Axes ? 
The eighth, which has been studied more than all the 

rest, is known to revolve once upon its axis during 
every periodic revolution ; from which it is inferred that 
they all revolve on their respective axes in the same 
manner. 




SATELLITES OP HERSCHEL 

618. How many Satellites has the planet Herschel 1 ? 

Six. 

[He is generally allowed to have six, upon the authority of Sir Wm. Herschel, and 
his son, Sir John Herschel. Only three, however, have ever been seen by any other 
observers, and seldom over two.] 

619. What are their Distances ? 
From 224,000 to 1,500,000 miles. 

620. Their Periodic Times ? 

From 1 day, 21 hours, to 111 days, according to their 
respective distances. 



130 PEIMAEY ASTKONOMY. 

621. What remarkable peculiarities do these Satellites exhibit ? 

Their orbits are nearly perpendicular to the ecliptic, 
and they revolve hackwa/rd, or from east to west, con- 
trary to all the other motions of our planetary system. 

622. Have we any other information respecting these Moons ? 

'None ; except that they revolve in orbits nearly cir- 
cular, and are described by Dr. Herschel as " the most 
difficult objects to obtain a sight of of any in our sys- 
tem." 



SATELLITE OP NEPTUNE. 

623. When and by whom was this Satellite discovered? 

By Mr. Lassell, of Liverpool, England, October 10, 
1846. 

624. What is its Distance from Neptune ? 

About 230,000 miles, or near the distance of our 
Moon. 

625. What is its Periodic Time % 
Five days and twenty-one hours. 

[We have here another illustration of the great law of planetary motion explained 
on page 48. So great is the attractive power of Neptune, that to keep a satellite, at 
the distance of our Moon, from falling to his surface, it must revolve some five limes 
as swiftly as she revolves around the Earth. The centripetal and centrifugal forces 
must be balanced in all cases.] 

626. Are there any suspicions that Neptune has other Satellites ? 

Professor Bond, of Cambridge, Mass., states that he 
has at times been quite confident of seeing a second 
satellite. 

627. What general fact has been arrived at with respect to the 
Secondary Planets ? 

That the laws of gravitation and planetary motion, 
discovered by Eewton and Kepler, extend to and pre- 
vail among all the secondaries. 



PKIMAKY ASTKO^OMY. 131 

LESSON XXX. 

OF TIDES. 

628. What are Tides? 

The alternate rising and falling of the waters of the 
ocean, at regular intervals. 

629. What is meant by Flood and Ebb Tides 1 

When the waters are rising, it is called Flood Tide ; 
and when they are falling, Ebb Tide, 

630. What is meant by High and Low Tide ? 

"When the waters have reached the highest and low- 
est points to which they usually go. 

631. Is this elevation and depression uniform as to its amount 1 
It is not : some high tides are much higher and others 

much lower than the average elevation. 

632. What are these extraordinary High Tides called 1 

Spring Tides. 

633. What are the remarkably Low Flood Tides called ? 
JVeap Tides. 

634. How often does the Tide ebb and flow ? 
Twice every day. 

[That is, we have two ebb and two flood tides every twenty-four hours, nearly.] 

635. What is the cause of the Tides ? 
The attraction of the Sun and Moon upon 

the waters of the ocean. 

[In this figure, the Earth is represented as surrounded by water, 
in a state of rest or equilibrium, as it would be were it not acted 
upon by the Sun and Moon.] 

636. What should we suppose would be the natural effect of the 
Moon's attraction ? 

To produce a single tide-wave on the side of the 
Earth toward the Moon. 





€ 



132 PRIMARY ASTRONOMY. 



[1. In this cut, the Moon is shown at a distance above the Earth, one tide-wave. 
and attracting the waters of the ocean so as to produce a high tide ^j^ 

at A. But as the Moon makes her apparent westward revolution H./ 

around the Earth but once a day, the simple raising of a flood tide 
on the side of the Earth toward the Moon, would give us but one 
flood and one ebb tide in twenty-four hours ; whereas it is known 
that we have two of each. 

2. "The tides," says Pr. Herschel, "are a subject on which 
many persons find a strange difficulty of conception. That the 
Moon, by her attraction, should heap up the waters of the ocean 
under her, seems to many persons very natural. That the same 
cause should, at the same time, heap them up on the opposite side of the Earth (as at 
B in the figure), seems to many palpably absurd. Yet nothing is more true."] 

637. Does the Moon cause but one Tide-Wave upon the globe ? 
She produces two at the same time ; one nearly under 

her, and the other on the opposite side of^^^^. 

" J:i: TWO TIDE-WAVES. 

the Earth. 

[In this cut we have a representation of the tide-waves as they 
actually exist, except that their hight, as compared with the magni- 
tude of the Earth, is vastly too great. It is designedly exaggerated, 
the better to illustrate the principle under consideration. While 
the Moon at A attracts the waters of the ocean, and produces a 
high tide at B, we see another high tide at C on the opposite side 
of the globe. At the same time it is low tide at D and E.] 

638. In what direction do these Tide-Waves move? 

From east to west, or as the Moon appears daily to 
revolve. 

[These four tides, viz. two high and two low, traverse the ocean from east to west 
every day, which accounts for both a flood and an ebb tide eveiy twelve hours.] 

639. How do you account for the Tide- Wave on the side of the 
Earth opposite the Moon ? 

It is due principally to the difference between the 
Moon's attraction on different sides of the Earth. 

[1. The student may do well to review the subject of gravitation, 195, 206, and 207. 

2. The diameter of the Earth amounts to about 30 th of the Moon's distance, so that 
by the rule, Question 206, the difference in her attraction on the side of the Earth 
toward her, and the opposite side, would be about ySth. The attraction being stronger 
at B (in the last cut) than at the Earth's center, and stronger at her center than at C, 
would tend to separate these three portions of the globe, giving the waters an elon- 
gated form, and producing two opposite tide-waves, as shown in the cut.] 

640. What other influence helps to produce the Tide-Wave oppo- 
site the Moon ? 

The revolution of the Earth around the common cen- 




PRIMARY ASTRONOMY. 133 



ter of gravity between her and the Moon during every 
lunation. 

641. What is meant by the Center of Gravity? 

The point between them where they would exactly 
balance each other if connected by a rod, and poised 
upon a fulcrum. 

CENTER OF GRAVITY BETWEEN THE EARTH AND MOON. 




642. Where is the Center of Gravity between the Earth and 
Moon situated! 

About 6,000 miles from the Earth's center. 

[This point is represented at A in the above cut, and also in the one following. We 
give 6,000 miles as the answer to the question, on the authority of Furguson. See 
his Note to Art. 298, London edition, 1764.] 

643. How does the Revolution of the Earth and Moon around the 
common Center of Gravity between them, help to produce a Tide- 
Wave opposite the Moon ? 

By generating an increased centrifugal force on that 
side of the Earth. 



CAUSE OF HIGH TIDE OPPOSITE THE MOON. 



€ 



[1. The point A represents the center of gravity between the Earth and Moon; 
and as it is this point which traces the regular curve of the Earth's orbit, it is 
represented in the arc of that orbit, while the Earth's center is 6,000 miles one side of 
it. Now the law of gravitation requires that while both the Moon and Earth revolve 
around the Sun, they should also revolve around the common center of gravity 
between them, or around the point A. This would give the Earth a third revolution, 
in addition to that around the Sun and on her axis. The small circles show her path 
around the center of gravity, and the arrows her direction. 

2. This motion of the Earth would slightly increase the centrifugal tendency at B, 
and thus help to raise the tide-wave opposite the Moon. But as this motion is slow, 
corresponding with the revolution of the Moon around the Earth, the centrifugal force 
could not be greatly augmented by such a cause.] 

__ __ 



134 PRIMARY ASTRONOMY. 

644. Which attracts the Earth most powerfully, as a whole, the 
Sun or the Moon ? 

The Sun. 

645* Which contributes most to the production of the Tides 1 

The Moon. 

646. Why is this? 

On account of her nearness, which makes a great 
difference in her attraction on different sides of the 
Earth. 

[1. It must be remembered that tides are the result, not of the attraction of the Sun 
and Moon upon our globe as a whole, but of that difference in their attracting forces, 
caused by a difference in the distance of the several parts. See Question 639, &c. 

2. The difference in the distance of two sides of the Earth from the Moon is 3 Q-th 
of the Moon's distance ; as 240,000 -\- 8,000 = 30 ; while the difference, as compared 
with the distance of the Sun, is only -rr.TF7"5 th > as 95,000,000 -f 8,000 = 11,875.] 

647. What is the comparative influence of the Sun and Moon in 
causing Tides ? 

As one to three / the Moon contributing three times 
as much as the Sun. 

64§. Does Flood Tide occur at the same hour each successive 
day? 

It does not. 

649. What variation is there ? 

It happens about 50 minutes later. 

650. Why is this? 

Because the Moon, which causes the tides, is re- 
volving eastward, and comes to the meridian 50 min- 
utes later each successive day. 

[As the two tide-waves are opposite each other, if the one next the Moon is later, 
the other also must be, as is found to be the case.] 

651. What is the Time between two successive High Tides ? 

Twelve hours and twenty-five minutes. 

652. Is it Flood Tide when the Moon is on the Meridian ? 
It is not. 

653. Why is it not? 



PRIMARY ASTROXOaEY. 



135 



Because the waters do not at once yield to the im- 
pulse of the Moon's attraction, but continue to rise 
after she has passed over. 

654. How far is the Flood Tide be- 
hind the Moon? tfY 

In the open sea it is generally '/&* 
about three hours, or 45° behind. / 



TIDE-WAVES BEHIND THE MOON. 




[In the cut the Moon is on the meridian, but the 
highest point of the wave is at A, or 45° east of 
the meridian ; and the corresponding wave on the 
opposite side at B is equally behind.] 

655. Do any other causes affect the 
time of High Tide ? 

It is affected by winds, and by the situation of dif- 
ferent places. 

[If a place is situated on a large bay, with but a narrow opening into the sea, the 
tide will be longer in rising, as the bay has to fill up through a narrow gate. Hence it 
is not usually high tide at New York till eight or nine hours after the Moon has 
passed the meridian.] 



LESSON XXXI. 



OF SPEIN T G- AXD NEAP TIDES. 



656. What is the cause of the Spring Tides? 

The combined influence of the Sun and Moon. 



CAUSE OF SPRING TIDES. 




:\ D \ 




. .»' 



[1. Here the Sun and Moon, being in conjunction, unite their forces to produce an ex- 



136 



PKIMAKY ASTEOl^OMT. 



SPRING AND NEAP TIDES. 



traordinaiy tide. The same effect follows when they are in opposition ; so that we 
have two spring tides every month ; namely, at New and Pull Moon. 

2. If the tide-waves at A and B are one-third higher at the Moon's quadrature than 
usual, those of C and D will be one-third lower than usual.] 

657. What is the cause of the Neap Tides % 

The Sun and Moon acting against each other. 

[1. On the right side of the cut the 
Sun and Moon are in conjunction, 
and unite to produce a spring tide. 

2. At the First Quarter their at- 
traction acts at right angles, and the 
Sun, instead of contributing to the 
lunar tide-wave, detracts from it to 
the amount of his own attractive 
force. The tendency to form a tide 
of his own, as represented in the 
figure, reduces the Moon's wave to 
the amount of one-third. See 

3. At the Full Moon she is in oppo- 
sition to the Sun, and their joint at- 
traction acting again in the same 
line, tends to elongate the fluid por- 
tion of the Earth, and a second spring 
tide is produced. 

4. Finally, at the Third Quarter the 
Sun and Moon act against each 

other again, and the second neap tide is the result. Thus we have two spring and 
two neap tides during every lunation ; the former at the Moon's syzyges, and the latter 
at her quadratures.] 




yiNi<-/ 



*>k^y 



<& p^. 




^APxt--' 



65§. Are all Spring Tides alike as to their elevation ? 
They are not : some are much higher than others. 
659. What is the cause of this variation ? 
The variation in the distances of the Sun and Moon. 



VARIATIONS IN THE SPRING TIDES. 



^ 



€ 







y 



[1. At A the Earth is in aphelion, and the Moon in apogee, and as both the Sun and 
Moon are at their greatest distance, the Earth is least affected by their attraction, and 
the spring tides are proportionally low. 



PRIMARY ASTRONOMY. 



137 



2. At B the Earth is in perihelion, and the Moon in perigee ; so that both the Sun 
and Moon exert their greatest influence upon our globe, and the spring tides are 
highest, as shown in the figure. In both cases the Sun and Moon are in conjunction, 
but the variation in the distances of the Sun and Moon causes variations in the spring 
tides.] 

660. What other general variation of the Tides has been noticed? 

That in Winter and Summer every alternate tide is 
higher than the intermediate one. 

661. What is the cause of this 1 

It is owing to the greater dec- 
lination of the Sun and Moon. 

[1. At the time of the equinoxes, the Sun being 
over the equator, and the Moon within 5£° of it, the 
crest of the great tide-wave will be on the equator ; 
but as the Sun and Moon decline south to A, one 
tide-wave forms in the south, as at B, and the oppo- 
site one in the north, as at C. If the declination 
was north, as shown at D, the order of the tides 
would be reversed. This subject may be still fur- 
ther illustrated by the following diagram : 



TIDES AFFECTED BY 
DECLINATION. 



D ® 



€): 




ALTERNATE HIGH AND LOW TIDES. 




2. Let the line A A represent the plane of the ecliptic, and B B the equinoctial. 

3. On the 21st of June the day tide-wave is north, and the evening wave south, so 
that the tide following about three hours after the Sun and Moon, will be higher than 
the intermediate one at 3 o'clock in the morning. 

4. On the 23d of December, the Sun and Moon being over the southern tropic, the 
highest wave in the southern hemisphere will be about 3 o'clock P. M., and the 
lowest about 3 o'clock A. M. ; while at the north this order will be reversed. It is 
on this account that in high latitudes every alternate tide is higher than the inter- 
mediate ones, the evening tides in Summer exceeding the morning tides, and the 
morning tides in Winter exceeding those of evening.] 

662. To what other variations are the Tides subject 1 

They are often hastened or retarded, and increased or 
diminished, by strong winds. 

663. What is the cause of the great variation of the same Tides in 
different places 1 

In some places the tide-wave is pressed into nar- 



12* 



138 PKIMARY ASTRONOMY. 

row bays or channels, which makes it rise much higher 
than at other places. 

[The average elevation of the tide at several points on our coast is as follows: 

Cumberland, head of the Bay of Fundy 71 feet. 

Boston 11$ " 

New Haven 8 " 

New York 5 " 

Charleston, S. C 6 " .] 

664. Have Inland Seas and Lakes any Tides ? 

None that are perceptible. 

665. Why is this? 

Because they are too small, compared with the whole 
surface of the globe, to be sensibly aifectecl by the at- 
traction of the Sun and Moon. 

666. How is the subject of the Tides generally regarded ? 

As a difficult one to be fully understood and ex- 
plained. 

[La Place, the great French mathematician and astronomer, pronounced it one of the 
most difficult problems in the whole range of celestial mechanics.] 

667. Is it likely that the Atmosphere has its Tides as well as the 
Waters ? 

It is probable that it has, though we have no means 
as yet for definitely ascertaining the fact. 



LESSON XXXII. 

OF COMETS. 
66§. What are Comets ? 

They are a singular class of bodies, belonging to the 
Solar System, distinguished for their long trains of 
light, their various shapes, and the great eccentricity of 
their orbits. 

669. From what is the term Comet derived? 

From the Greek word coma, which signifies hair; on 



PRIMARY ASTRONOMY. 



139 



account of the bearded or hairy appearance of some 
comets. 

670. Are Comets Self-luminous, or Opake ? 

They are known to be opake, from the fact that they 
sometimes exhibit phases, which show that they shine 
only by reflection. 

671. How are Comets usually distinguished one from another ? 

By the date of their appearance, or by a specific 
name given to them. 



[Thus we have the comets of 1585, 1680, 1811, &c 
Encke's Comet, Biela's Comet, &c] 

672. What are the 'princi- 
pal parts of a Comet ? 

The nucleus, the envel- 
ope, and the tail. 

673. What is the Nucle- 



and also Bailey's Comet, 

GREAT COMET OF 371 BEFORE CHRIST. 




It is the most dense or 
solid portion, sometimes 
called the head. (See E 
in the cnt.) 

674. What is the Envel- 
ope ?f 

A thin misty wrapper or covering surrounding the 

nucleus. (See E in the cut.) 

675. What is the Tail of a Comet ? 

An expansion or elongation of the envelope, extend- 
ing off in one direction from the nucleus. (See T in 
the cut.) 

676. Have all Comets these three parts ? 

* No'-cle-us, the kernel or nut ; the central part of any body about 
which matter is collected. The plural of this term is nu-cle-i. 
f En-vel'-ope, a wrapper or inclosing covering. 



140 



PRIMARY ASTRONOMY. 



COMKT (IF IPS." 




They have not. Some have 
a nucleus and no envelope ; 
some no perceptible nucleus ; 
and others a nucleus and envel- 
ope, but no tail. 

[A comet that appeared in 1585 had simply 
an envelope, as shown in the cut. Encke's, 
comet is another of this kind. See cut, page 
143. In 1682 one was seen as round and bright 
as Jupiter, without even an envelope. But these 
are rare exceptions to the general character of 
comets.] 

$*Z*t* Hoiv are the Tails of Comets usually situated? 

They extend in a direction opposite the Sun. 

[This is true, not only when going toward the Sun, but also when going from him. 
See cut upon the opposite page.] 

67§. What is their usual rOMET ny 1744 

Form? 

They assume a great va- 
riety of shapes : some ap- 
pearing like an enormous 
fan; others like a long 
sword or saber; but all 
curved more or less, and 
concave toward the regions 
from which they come. 

[The comet of 1744, represented in this 
cut, excited great attention and interest. It 

exhibited no train till within the distance of the orbit of Mars from the Sun ; but 
early in March it appeared with a tail divided into six branches, all diverging, but 
curved in the same direction. Each of these tails was about 4° wide, and from 30° to 
44° in length. The edges were bright and decided, the middle faint, and the inter- 
vening spaces as dark as the rest of the firmament, the stars shining in them. When 
circumstances were favorable to the display of this remarkable body, the scene was 
striking and magnificent, almost beyond description. Milner's " Tour through Crea- 
tion."] 

679. What is the Form of their Orbits ? 
They are generally very elliptical. 

[The form of a comet's orbit is represented on the opposite page.] 




PRIMARY ASTRONOMY. 141 



ORBIT OF A COMET. 



[Here it will be seen that the orbit is very eccentric, that the perihelion point is 
very near the Sun, and the aphelion point very remote. See cuts, pages 2 and 30.] 

6§0. What effect has this Eccentricity upon the motion of Comets ? 

It makes a great difference in their velocity in differ- 
ent parts of their orbits. (See first cnt, page 70, and 
Note.) 

€81. What can you say of their Motions when near the Sun ? 

When passing their perihelion their velocity is some- 
times inconceivably rapid. 

[1. The comet that appeared in 1472 described an arc of 120° in the heavens in a 
single day. That of 1680 moved at the rate of 1,000,000 miles an hour ! 

2. How so light a body can be made to pass through space with such velocity is in- 
conceivable ; but we should remember that the space through which they pass is not 
filled with air, like the regions of our globe, but is utterly void or empty.] 

6§2. What effect has the change of position upon their appear- 
ance? 

Their tails usually increase both in length and breadth 
as they approach the Sun, and contract as they recede 
from him. 

[This elongation and expansion, however, may be merely apparent. As the comet 
approaches or retires from the region of the planets, their heads are nearly toward the 
Earth ; but when within the orbit of Jupiter, or about the Sun, we often have a side 
view of them, under which circumstances they would, of course, appeal- much larger. 
By observing the last cut, the student will easily see how a comet might contract as it 
approaches the Sun, as it seems to in the cut, and yet appear much larger when in 
his neighborhood, than when first seen at a distance.] 

683. What other peculiarity has been noticed? 

The tail of the comet of 1811 is said to have ex- 
panded suddenly to a great distance. 



142 



PRIMARY ASTRONOMY. 



GREAT COMET OF 1811. 




6 §4. How are the Orbits of Comets situated with respect to the 
Ecliptic ? 

They approach the Sun from every point of the 
heavens, all around and on both sides of the ecliptic. 

[Some comets seem to come up from the immeasurable depths below the ecliptic, 
and having passed their perihelion, plunge off again into space, and are lost for ages in 
the ethereal void. Others appear to come down from the zenith of the universe, and 
having passed around the Sun, reascend far above all human vision.] 

685. Is any thing known of their Temperatures 1 
Only that some approach very near the Sun, and 
must therefore become very hot. 

[The comet of 1680 came within 130,000 miles of the Sun's surface, and must have 
received 28,000 times the light and heat which the Earth receives from the Sun— a 
heat more than 2,000 times greater than that of red-hot iron !] 

6S6. What can he said of the Size of Cornets 1 

Their nuclei or heads are comparatively small, being 
only from 33 to 2,000 miles in diameter. Their tails 
are often of enormous length. 

[1. The comet of 371 B. C. (page 139) had a tail 60° long, covering one-third part 
of the visible heavens. It was estimated at 140 millions of miles in length. 

2. The comet of 1680 was 70° in length, estimated at 100 millions of miles. Though 
its head set soon after sundown, its tail continued visible all night. 

3. In 1618 a comet appeared which was 104° long. Its tail had not all risen when 
its head had reached the middle of the heavens. 

4. The comet of 1843 was 60°, or 130,000,000 miles in length.] 



PKBfAEY ASTRONOMY. 



143 



GREAT COMET OF 1843. 



687. Is any thing known of the Physical Nature of Comets? 

They are known to be exceedingly light vapor or 
gas. 

6§§. How is this known 1 

From the fact . that the fixed stars have been seen 
through their densest portions. 

6§9. Are they subject to the Law of Gravitation 1 

They are ; bnt are so light as to have no sensible 
effect npon the planets. 

[1. While Jupiter and Saturn often retard and delay comets for months in tneir 
periodic revolutions, comets have not power in turn to hasten the time of the planets 
for a single hour. The comet of 1770 got entangled, by attraction, among the l\f oons 
of Jupiter, on its way to the Sun, and remained near them for four months ; yet it did 
not sensibly affect Jupiter or his Moons. This shows conclusively that the relative 
masses of the comets and planets are almost infinitely disproportionate. 

2. The fact that they revolve about the Sun, is a sufficient proof of their being sub- 
ject to the great law of gravitation.] 



and 
to 



690. What is known of the 
Periodic Times of Comets ? 
Only four or five have 
been ascertained ; 
these vary from ! _ 
570 years. 

[The following periodic revolutions 
have been fully determined : 

Encke's Comet 3i years. 

Biela's " 6£ " 

Halley's " 76 « 

Comet of 1680 570 " 

The next return of this last will be in the year 2.250.] 



ENCKK'S COMET. 




144 PRIMAEY ASTRONOMY. 

691. Are all Comets supposed to revolve continually around the 
Sun? 

Professor Mchol and Sir John Herschel are of opin- 
ion that the greater number visit our system but once, 
and then fly off in nearly straight lines, till they pass 
the center of attraction between the Solar System and 
the Fixed Stars, and go to revolve around other Suns in 
the far-distant heavens. 

692. What can we say of the Distance to which many Comets go ? 
In some cases it must be immense, from the time they 

are gone, and the rapidity of their motions. 

[1. The orbit of Encke's comet is orbits of several comets. 

wholly within the orbit of Jupiter, w 

while that of Biela's extends but a ...-••' " "--... 

short distance beyond it. The aphelion "y ©•'' ""••... 

distance of Halley's comet is 3,400 mill- ,.'" "> ?j 

ions of miles, or 550 millions of miles / ^ncke's '.. 

beyond the orbit of Neptune. But / , ..■ ®- ' « ~"W -.^ 

these are all comets of short periods. „....•' H^. L . L .^ Y ..^.?6'y/? s .,-•'' J \ 

2. Though the distauce to which : .-Z-^y:;-*^^^ ■ \ 

some comets go, to be so long absent, ; ;.'■': ^ \\ ^S^._ ..•■" \ 

must be very great, still their bounds ; ;' ffi '^'..;- •'' ' • 

are set by the great law of attraction ; \ \ •-'.'■' \ / 

for were they to pass the point " where \ \ / \ / 

gravitation turns the other way," they 
would never return. But most, if not 
all, do return, after their ' ; lon^ travel of 



a thousand years." What a sublime >:. M ''''^ho '''' 

conception this affords us of the almost ..••*' V' 1 • --'"•-"?. ."' 

infinite space between the Solar System y^ 
and the Fixed Stars ! 

3. The student will find the entire orbit of a comet represented in the second cut on 
page 30. The aphelion point is represented as only about half way to the Fixed 
Stars.] 

693. How were Comets regarded by the ancients ? 

As harbingers of famine, pestilence, war, and other 
dire calamities. 

[The comet of 1811 was regarded by the ignorant as the precursor of the war that 
was declared in the following spring. In one or two instances comets have excited 
serious apprehension that the day of judgment was at hand; and that they were the 
appointed messengers of Divine wrath, hasting apace to burn up the world. This was 
the case with a large comet that appeared in 1456.] 

694. What other fears have been entertained with regard to 
Cornels ? 



PKIMARY ASTKO^OMY. 145 

That they might come into collision with our globe, 
and either dash it to pieces, or burn every thing from its 
surface. 

695. Is there really any danger of collision ? 

None at all ; the thing is next to impossible. 

[1. It has been determined, upon mathematical principles, and after the most ex- 
tended and laborious calculation, that of 281,000,000 of chances, there is only one un- 
favorable, or that can produce a collision between the two bodies. 

2. When we consider the order and harmony that prevail throughout the planetary 
system, and remember that the same infinitely wise and powerful Being that guides 
the planets in their courses, marks also the pathway of every comet, it is not easy to 
admit that they are plunging through the system at random, and are liable to come in 
collision with the planets. It would argue a want of design and perfection in the 
mechanism of the heavens, which would be a reflection upon the Divine Architect.] 

696. Would it be destructive if a collision were actually to take 
place ? 

Probably not. Comets are generally too light even 
to penetrate our atmosphere to the Earth's surface. 

[1. The air is to us what the waters are to fish. Some fish swim around in the 
deep, while others, like lobsters and oysters, keep on the bottom. So birds wing the 
air, while men and beasts are the " lobsters" that crawl around on the bottom. Now 
there is no more probability that a comet would pass through the atmosphere, and 
injure us upon the Earth, than there is that a handful of fog or vapor thrown down 
upon the surface of the ocean, would pass through and kill the shell-fish at the 
bottom. 

2. Professor Olmsted remarks that, in the event of a collision, not a particle of the 
comet would reach the Earth— that the portions encountered by her would be 
arrested by the atmosphere, and probably inflamed ; and that they would perhaps ex- 
hibit, on a more magnificent scale than was ever before observed, the phenomena of 
shooting stars, or meteoric showers.] 



LESSON XXXIII. 

OF THE SUN. 

697. How is the Sun distinguished? 

As the great center of the Solar System, the fountain 
of light and heat. (See also 131.) 

698. What names did the ancients give to the Sun ? 

The Eomans called him Sol, and the Greeks Helios. 
(See Notes to 126 and 305.) 




146 PEIMAEY ASTBONOMY. 



699. What did they suppose him to be ? 
A vast globe of fire. 

[It is by no means strange that this opinion should obtain among the ancients with 
respect to the Sun. It has even been held by some modern astronomers, among whom 
is the celebrated and profound La Place. This opinion, however, is now almost uni- 
versally rejected. The heat produced by the light of the Sun is found not to be trans- 
mitted from him, but to be produced by the contact of the rays with other substances ; 
and greatly modified by the relative density of the atmosphere.] 

700. What is his Magnitude? 

He is 886,000 miles in diameter. 

[1. The vast magnitude of the Sun sun rising in the distance behind 

may be inferred from the fact that when a church. 

rising or setting he often appears larger 
than the largest building, or the tops of 
the largest trees. Now if the angle filled 
by him at the distance of two miles is 
over 100 feet across, what must it be at 
the distance of 95 millions of miles ? 

2. Were a railroad passed through the 
Sun's center, and should a train of cars 
start from one side, and proceed on at 
the rate of 30 miles an hour, it would 
require 3? years to cross over his diameter. To traverse his vast circumference, 
at the same rate of speed, would require nearly 11 years. 

3. The Earth's diameter is 7,912 miles; and yet it would take 112 such globes, if 
laid side by side, to reach across the diameter of the Sun : 886,000 ~ 7,912 = 112, 
nearly.] 

701. What is his Magnitude or mass as compared with our 
globe ? 

He is equal to 1,400,000 such worlds. 

[1. For comparative magnitudes of the Sun and planets, see cut, page 43. 
2. The student will bear in mind that the magnitudes of spherical bodies are not in 
proportion to the diameters, but to the cubes of their diameter. See Note after 187.] 

702. How does the Sun compare, as to size, with the rest of the 
system ? 

He is 500 times larger than all the other bodies of 
the system put together. 

[This estimate includes all the planets, primary and secondary, but has no reference 
to comets.] 

703. How does he compare with the size of the Moon's orbit? 

If his center occupied the position of the Earth, he 
would fill the whole orbit of the Moon, and extend 
more than 200,000 miles beyond it in every direction. 



PRIMAKY ASTRONOMY. 



147 



[The mean distance of the Moon from the the sun, and the moon's orbit. 
Earth's center is 240,000 miles ; consequently 
the diameter of her orbit, which is twice the 
radius, is 480,000. Subtract this from 886,000, 
the Sun's diameter, and we have 406,000 
miles left, or 203,000 miles on each side, be- 
yond the Moon's orbit] 

704. How does the Sun appear 
through a Telescope ? 

Like a vast globe of fire, 
with dark spots here and there 
upon its surface. 

705. What is the number of these Spots ? 

It varies at different times from two or three to fifty. 




TELESCOPIC VIEW OF THE S 




[1. Much depends, of course, upon 
the power of the instrument through 
which he is viewed ; as some telescope? 
will reveal much more than others. 

2. For several days, during the latter 
part of September, 1846, the author 
could count sixteen of these spots which 
were distinctly visible, and most of them 
well defined ; but on the 7th of October 
following, only six small spots were 
visible, though the same telescope was 
used, and circumstances were equally 
favorable. 

3. The Sun is a difficult object to view 
through a telescope, even when the eye 
is protected in the best manner by 
colored glasses. In some cases (as in 
one related to the author by Professor 
Caswell, of Brown University) the heat 

becomes so great as to spoil the eye-pieces of the instrument, and sometimes the eye 
of the observer is irreparably injured.] 

706. Do these Spots appear stationary, or in motion ? 

They appear to pass over the Sun from left to right 
in about 13| days. 

707. What has been inferred from this fact ? 

That the Sun revolves on his axis, from west to east, 
or in the direction of all the planets, every 25^ days 
(25 d. 10 h.) 

[I. This is the time of his sidereal or true revolution. His apparent or synodic revo- 
lution requires 27 days, 7£ hours ; but this is as much more than a complete revolution 
upon his axis, as the Earth has advanced in her orbit in 25£ days. Let S represent 



J 



148 



PRIMARY ASTRONOMY. 



SIDEREAL AND SYNODIC REVOLUTIONS OF THE SUN. 



-•■$m 



ffiL 



?5a 



ip H 



S YW 



db\c 



9.T.V. 



llW.v 




the Sun, and A the Earth in her 
orbit. When she is at A, a spot 
is seen upon the disk of the Sun 
at B. The Sun revolves in the 
direction of the arrows, and in 
25 days, 10 hours, the spot comes 
round to B again, or opposite the 
star E. This is a sidereal revo- 
lution. 

2. During these 25 days, 10 
hours, the Earth has passed on 
in her orbit some 25°, or nearly ^f 
to C, which will require nearly 
two days for the spot at B to get 

directly toward the Earth, as shown at D. This last is a synodic revolution. It con- 
sists of one complete revolution of the Sun upon his axis, and about 27° over.] 

70§. Where are these Spots situated ? 

They are usually on each side of the Sun's equator, 

and within a zone of 60°. 

709. How is the Sun's Axis situated with respect to the Ecliptic 1 
It is inclined toward it 7° 20'. 

710. How was this inclination ascertained ? 

By observing changes in the direction of the solar 
spots, at different seasons of the year. 

VARIOUS DIRECTIONS OF THE SOLAR SPOTS. 

C. D. 




March. 



June. 



September. 



[1. Let E F represent the plane of the ecliptic. In March the spots describe a curve, 
which is convex to the south, as shown at A. In June they cross the Sun's disk in 
nearly straight lines, but incline upward. In September they curve again, though in 
the opposite direction ; and in December pass over in straight lines, inclining down- 
ward. 

2. The figures B and D show the inclination of the Sun's axis.] 

711. How does this prove that the Sun's Axis is inclined? 
If the Sun's axis were perpendicular to the ecliptic, 
the spots would revolve in circles parallel to the eclip- 



PEIMARY ASTRONOMY. 149 

tic, and apparently in straight lines ; whereas the in- 
clination of his axis would give the spots precisely these 
motions during the year. 

[This subject will be fully understood by consulting the following figure, in con- 
nection with the preceding : 

SOLAR SPOTS OBSERVED FROM DIFFERENT POINTS. 



DEC. 




1. If the Sun's axis were at right angles with the ecliptic, his equator and the spots 
upon his disk must revolve parallel to the ecliptic, and would appear to cross his disk 
in straight lines, from all parts of the Earth's orbit, or throughout the year. 

2. In March, however, the spots move in curve lines, as represented in the cut, show- 
ing that the North Pole of the Sun inclines toward us. 

3. In June we have a side view of the Sun's axis, and the spots seem to pass upward 
in straight lines, as represented at B on the opposite page. 

4. In September the South Pole of the Sun inclines toward us, and the spots again 
move in curve lines, the reverse of what they were six months before. 

5. In December we have another side view of the axis, and the spots cross in straight 
lines, inclining downward, as shown in the opposite figure at D.] 

712. What is the Size of the Solar Spots? 

They are of various sizes and forms, some having 
been estimated at 50,000 miles across. 

713. What is their general appearance? 

They are darkest in the middle, and are shaded off 
at the edges by a sort of penumbra. 

714. Do they appear of the same size throughout their whole 
course ? 

They appear narrow when first seen on the Sun's 
eastern limb ; expand gradually till they reach the cen- 
ter ; and then contract till they pass off on the west. 

715. What is the cause of this apparent expansion and con- 
traction ? 

13* 



150 PRIMAKY ASTRONOMY. 

It is because the spots are on the surface of a globe, 
and are seen partly edgewise, except when near the 
Sun's center. 

716. Is their rate of motion across the Sun uniform ? 

It is not] but is accelerated from the eastern limb to 
the center, and retarded from the center to the western 
edge. 

717. What are these Spots supposed to he? 

They are generally thought to be openings through 
the luminous atmosphere of the Sun. 

[Some have thought them to be the tops of mountains, laid bare by tides, or other 
fluctuations of the solar atmosphere.] 

718. What is the prevailing opinion in regard to the nature of 
the Sun ? 

That his body is opaJce / and that his light proceeds 
from a luminous atmosphere that surrounds him. 



THE ZODIACAL LIGHT. 

719. What is the Zodiacal* Light? 

It is a faint nebulous light, resembling the tail of a 
comet, sometimes seen in the neighborhood of the Sun. 

720. At what time may it be seen ? 

Just after sundown or before sunrise in March, April, 
October, and November. 

721. What is its Appearance ? 

It is quite faint, hardly distinguishable from ordinary 
twilight. 

722. What is its Form and Position ? 

It has the form of a pyramid, with its base toward 

* Zo-Dl'-AC-AL. 



PRIMARY ASTRONOMY. 



151 



ZODIACAL LIGHT. 



the Sun, and inclines a 
little toward the horizon. 

723. How far does it extend 
from the Sun ? 

From forty to ninety 
degrees. 

724. What is its width at 
the base 1 

It varies from eight to 
thirty degrees. 

725. How is this substance 
situated with respect to the Sun's 
equator ? 

Its major axis is at right angles with the axis of the 
Sun. 




FORM, EXTENT, ETC., OF THE ZODIACAL 
LIGHT. 



[Let A represent the Sun, B B his 
axis, then C C will represent the extent, 
and D D the thickness of this curious 
appendage to the Sun.] 

726. What is this Light 
supposed to be ? 

Some have thought it 
an extension of the Sun's 
atmosphere, while others 
have regarded it as nebu- 
lous vapor, of the nature 
of comets. 

727. Is it thought to be at 
rest, or in revolution ? 

It is believed to revolve with the Sun on his axis, 
and to be flattened out as we see it, by that revo- 
lution. 

[See the effect of the revolution of yielding bodies upon their figure illustrated, 
page 61. The form is supposed to be that of a lens, of which the above is an edgewise 
vjew.] 

72§. What other motion has the Sun besides that on its Axis? 




152 PEIMARY ASTEONOMY. 

A slight periodical revolution around the common 
center of gravity of the Solar System. 

[This motion resembles that of the Earth, illustrated on page 133, to which the 
student is referred. The Sun never deviates from his apparent fixed position to the 
amount of more than twice his diameter.] 

729. Has he still another motion ? 

He is found to be revolving, with all his retinue of 
planets and comets, in a vast orbit, around some distant 
and unknown center. 

[1. This supposed orbit is represented in the second cut on page 30, to which the 
student will do well to turn. 

2. Professor Miidler, of Dorpat, in Russia, has recently announced as a discovery 
that the star Alcyone, one of the seven stars, is the center around which the Sun and 
Solar System are revolving. 

3. What a stupendous idea! Secondaries revolving around primaries; primaries 
around the Sun ; and the Sun around some other center ; and so on, till we come to the 
center of all other centers ; or, as Dr. Dick remarks, "to the throne of God!"] 

730. What is the estimated Velocity of the Sun and Solar Sijs- 
tem? 

About 28,000 miles per hour, or 8 miles per second. 

731. What is the supposed Period of Revolution ? 

About 18,200,000 years. 

[If this be correct, he has only passed over one 3,000th part of his orbit, or about 
seven degrees since the creation of the world— an arc so small compared with the 
whole, as to be hardly distinguishable from a straight line.] 



LESSON XXXIY. 

GENERAL REMARKS UPON THE SOLAR SYSTEM. 

732. How did the Solar System originate ? 

The Scriptures say that "in the beginning God 
created the heaven and the earth" (Gen. i. 1). 

733. Describe the Nebular Theory of Creation. 

It teaches that the elements or matter, of which the 
system is composed, was originally a vast cloud of 
vapor or mist, which has been condensed and formed 



PRIMARY ASTRONOMY. 153 

into Sun and planets, during a vast period of time, by 
the simple law of gravitation. 

734. Is this theory correct, or even probable 1 
It is not. 

735. What objections can be urged against it ? 

1. It would make the creation, mentioned by Moses, 
a mere organization or arrangement of pre-existing 
matter ; whereas the Bible says that " the worlds were 
framed by the word of God, so that things which are 
seen were not made of things which do appear" — Heb. 
xi. 3. 

2. If it allows that God created the materials of 
the system at all, it throws the period of their creation 
back indefinitely into eternity, and substitutes gravita- 
tion for the direct agency of the Almighty. 

736. What led to the discovery of the first Asteroid? 

The suspicion that there was a large planet in the 
space between the orbits of Mars and Jupiter. 

737. What singular opinion has been entertained in regard to 
their origin 1 

Doctor Olbers, of Bremen, Germany, was of opinion 
that they were originally one large planet, that had 
been broken into fragments by explosion, or by coming 
in collision with some other body. 

738. What says Dr. Herschel of this theory ? 

He says it may serve as a specimen of the dreams in 
which astronomers, like other speculators, occasionally 
and harmlessly indulge. 

739. Why is this theory improbable ? 

Because such an explosion or collision would be at 
variance with the harmony and order that every where 
prevail throughout the planetary regions. 



154 PEIMARY ASTEOKOMY. 

740. Is it probable that the planets are inhabited by rational 
beings ? 

It is. 

741. Have we any direct evidence of this fact? 

We have not : no inhabitants have ever been seen, 
heard, or heard from, upon any of the planets. 

742. Is this any proof that they do not exist there 1 

It is not. We must not conclude that a newly dis- 
covered island is uninhabited, because it is so far dis- 
tant when first seen that we cannot see or hear the 
people. 

743. Why is it believed that the planets are inhabited ? 
Because they are globes like our Earth ; and have 

atmospheres, seasons, days and nights, satellites, &c, 
which would be unnecessary if they were uninhabited. 

[To create twenty primary planets, and have only one small one inhabited, would be 
like a father's building twenty houses, all after one model, but of different colors and 
dimensions, and after having furnished them all with ventilators., mirrors, lamps, &c, 
to put an only son into one of the smallest, and leave the remaining nineteen unoc- 
cupied.] 

744. Do not the extremes of Cold and Heat upon the several 
planets forbid their being inhabited? 

By no means. The Creator adapts every creature to 
the place where he designs it to dwell. 

[Fish have cold blood ; and some animals may be frozen stiff, and when thawed out 
will come to life. The lion and polar bear are each adapted to their respective abodes, 
and so with every thing in nature. And why may not the same law extend to the 
planets ? Cannot He, who adapted the three Hebrews to the fiery furnace (Dan. iii. 
27), adapt beings to the temperature of Mercury ? Upon the same principle beings 
may exist even upon Neptune, to whom a milder climate would be uncomfortable.] 

What part of the book have you now gone over ? 

Parts First and Second, including Preliminary Ob- 
servations and Definitions ; and what relates to the 
Solar System. 

What yet remains to be examined ? 

Part Third, which relates to the Sidereal Heavens. 



PRIMARY ASTRONOMY. 155 

PART III. 
THE SIDEREAL HEAVENS. 



LESSON XXXY, 

THE FIXED STAES- 

TUDES, ETC. 

745. What is meant by the Sidereal Heavens (127) ? 

746. Why are some Stars called Fixed Stars? 

Because they occupy the same positions with respect 
to each other from age to age, while the planets are 
seen to be in motion. 

747. In what other respect do the Fixed Stars differ from the 
planets ? 

They are self-luminous, and seem to twinkle or scin- 
tillate, while the planets appear tranquil and serene. 

748. How may a Fixed Star be distinguished from a planet by 
the aid of a Telescope ? 

The planets exhibit a round, mild-looking disk, while 
the Fixed Stars appear only as a point of brilliant light. 

749. How are the Fixed Stars situated with respect to the Solar 
System ? 

They are inconceivably distant, and surround our 
system in every direction. 

[Were it not for the Sun, we should see the stars in the day-time as well as in the 
night. See 557.] 

750. What is the estimated distance of the Fixed Stars? 

The nearest are supposed to be 20,000,000,000 {twenty 
billions) of miles from the Sun, or more than 7,000 times 
as far off as Neptune. 



156 PEIMAEY ASTKONOMY. 

[1. For light to pass over this space, at the rate of 200,000 miles per second, would 
require upward of three years. 

2. Were the Earth's orbit one vast circle of light, it would not appear larger than a 
lady's finger-ring from the nearest Fixed Star.] 

751. How would our Sun appear from such a distance? 
Only like a bright star. 

752. What are the Fixed Stars supposed to be ? 

Distant Sims, and centers of other planetary systems. 

■753. How, then, should we regard our own Sun? 

As one of thousands of Suns, but appearing vastly 
more brilliant than the rest, solely because of our near- 
ness to him. 

754. How do the Stars appear in the heavens ? 

They seem to be equally distant from us, and scat- 
tered at random over the concave firmament. , 

755. What can you say of their apparent Size ? 

They vary from the large and bright star to the 
smallest that the eye can discover. 

756. What is the cause of this variation ? 

It is due, in a great measure, to the variation in their 
distcmces. 

757. How are the Stars classified ? 

They are first arranged in groups, or patches, called 
constellations. 

758. How are the Constellations distinguished ? 

They are named after some animal or object which 
the ancients imagined them to resemble. 

759. Into how many constellations are the heavens divided ? 

Ninety-three. 

760. How are they situated ? 

Twelve in the Zodiac, 35 north, and 46 south of it. 

761. Can you name one of the most conspicuous in each of these 
divisions ? 



PEIMAEY ASTKOKOMY. 157 



The Great Bear in the north ; Taarus in the Zodiac ; 
and Orion just south of the Zodiac. 

[If the student will look up these three in the heavens, it will form a good begin- 
ning, and will be of great service when he comes to take up the study anew, with a 
more advanced text-book.] 

762. What is the second step in classifying the Stars? 

They are divided into twelve classes, according to 
their apparent magnitudes. 

STARS OF DIFFERENT MAGNITUDES. 



>)c # * * * 



* * * * 



763. How many of these can be seen by the naked eye ? 

Only the first six classes. The remaining six are 
seen only by the aid of telescopes, and are called Tele- 
scopic Stars. 

764. What is the third step in classifying the Stars ? 

To classify the stars in each constellation by the use 
of the Greek alphabet, calling the largest alpha (a), 
the next largest beta (/3), &c "When the Greek alpha- 
bet is exhausted, the Roman is taken up ; and when this 
fails, recourse is had to figures. 

765. Is there any other method by which particular Stars are 
designated ? 

Many have specific names, as Arcturus, Sirius, AV- 
debaran, &c. 

766. What is the estimated number of the Fixed Stars ? 

!No finite mind can number them, but estimates have 
been made amounting to near 400 millions. 

[The Psalmist asserts the infinite knowledge of God, by saying that " He telleth the 
number of the stars, and calleth them all by their names." — Psalm cxlvii. 4.] 

767. In what proportion do the several Magnitudes occur 1 

There are but few of the first magnitude, and the 
number increases rapidly as the magnitudes diminish. 

14 



158 



PRIMARY ASTRONOMY. 



[The number of stars down to the twelfth magnitude, has been estimated as follows : 



Visible to 


1 2d 
J 3d 


the naked 

eye, 


I 4th 
I 5th 




t 6th 



f 1st magnitude 18 

52 

177 

376 

1,000 

4,000 

5,623 



Visible only 
through tel- 
escopes, 



7th magnitude 

8th " 

9th " 

10th " 

11th " 

1 12th *• 



26,000 

170,000 

1,100,000 

7,000,000 

46,000,000 

300,000,000 



Total number. .354,301,623] 



l 76§. Why are there so many more of the small Stars than of the 



large ones ? 

It is because we are in the midst of a great cluster, 
with but few stars near us, the number increasing as 
the circumference of our view is enlarged. See second 
cut, page 30. 

[1. We have here a representa- number of stars of each magnitude. 

tion of a great cluster of stars. Let 
the central star represent the Sun 
(a star only among the rest), with 
the Solar System revolving be- 
tween him and the first circle. 
The 18 stars in space 1st will ap- 
pear to be of the first magnitude, 
on account of their nearness, and 
♦hey are thus few because they 
embrace but a small part of the en- 
tire cluster. The stars of space 2 
will appear smaller, being more dis- 
tant, but as it embraces more space, 
they will be more numerous. Thus 
as we advance from one circle to 
another, the apparent magnitude 
constantly diminishes, but the num- 
ber constantly increases. The large 
white circle marks the limit of our 
natural vision. 

2. Even this cut fails to present fully to the eye the cause of the rapid increase in 
numbers; for we can only show the surface of a cut section of our firmament of stars, 
which exhibits the increase in a plane only ; whereas our Sun seems to be imbedded 
in the midst of a magnificent cluster, the stars of which we view around us in every 
direction.] 

769. Is any thing known of the actual Magnitude of the Stars ? 
Nothing very definite, though many of them are 
estimated to be much larger than our Sun. 

[The diameter of some of the Fixed Stars has been estimated at 200,000,000 miles, 
or more than 200 times the diameter of the Sun.] 




PEIMARY ASTRONOMY. 159 



LESSON XXXVI. 

OF DOUBLE, VARIABLE, AND TEMPORARY STARS. 

770. What are Double Stars ? 

Such as appear single to the naked eye, but when ex- 
amined by the telescope are found to be double. 

[1. The North Pole Star appears like a small single star to the naked eye, but with a 
telescope is found to consist of two. 

2. In many cases, what appears to be a single star is found to consist of from three to 
six, and even more.] 

771. Is it likely that all Stars that appear double are actually 
near each other ? 

It is not. Probably many appear near each other 
simply because they are near the same line of vision. 

WHY STARS MAY APPEAR DOUBLE. 

Apparent positions. True positions. 

1—--^= ::::::"=:;: * 

A B 

[Here the observer on the left sees a large and small star at A, apparently near 
together ; the lower star being much the smallest. But instead of their being situated 
as they appear to be, with respect to each other, the true position of the smaller star 
may be at B instead of A ; and the difference in their apparent magnitudes may be 
wholly owing to the greater distance of the lower star.] 

772. In what sense are Stars said to be double when one is far 
beyond the other ? 

They are said to be optically double. 

773. How many Double Stars are to be met with in the Heavens ? 
It is supposed there are not less than six tliousand. 

774. What suspicion did the great number of Double Stars 
awaken in the minds of astronomers 1 

That such stars were specially connected by gravita- 
tion. 

775. What surprising fact has been ascertained in regard to some 
of the Double Stars? 



160 PEIMAEY ASTEONOMY. 

That they are actually revolving one around another, 
or both around the center of gravity between them. 
776. How do we distinguish Double Stars that are thus con- 



As being physically double. 

777. What are these Systems called? 
Binary Systems. 

[These, it must be remembered, consist of one or more Suns revolving as described, 
but so distant as to appear only as stars.] 

778. Are there many of these Binary Systems ? 

Sir William Herschel noticed about fifty instances of 
changes in the relative position of double stars, and the 
revolution of some sixteen he considered certain. 

779. In what time do they revolve? 

From forty to twelve hundred years. 

780. What are Variable Stars ? 

Such as undergo a regular periodical increase and 
diminution of light. 

781. What are the causes of these variations? 

They are not known. It is thought they may be less 
luminous on one side than the other; and, by turning on 
their axes, may vary in brilliancy on that account ; or 
that planets revolving near them may cut off a portion 
of their light at regular intervals. 

782. What are Temporary Stars? 

Such as have disappeared from the heavens, and such 
as shine out suddenly, in a place previously void, as 
though just created. 

[Some writers classify these under the head of New and Lost Stars." 

783. Are these sudden appearances and disappearances fre- 
quent ? 

Ten new stars have appeared, and thirteen old ones 
seem to have perished, during the last hundred years. 



PKEVIAKY ASTRONOMY. 161 

784. How have some Christian writers regarded these sudden dis- 
appearances of Stars? 

As the terminations of probationary periods, like the 
conflagration that is to take place upon our own globe 
at the end of time. (See 2 Peter iii. 7, 10.) 



LESSON XXXYII. 

CLUSTERS OF STARS AND NEBTJLJB. 

785. What are Clusters of Stars ? 

They are patches in the heavens where the stars are 
unusually thick or near together. 

786. Can you name any specimens of Clusters ? 

The Seven Stars, or Pleiades, and the Uyades just 
east of them. 

787. Are these Clusters numerous? 

With a telescope many hundreds may be seen. 

788. How do they appear through a a cluster of stars. 
Telescope ? 

They are found to consist, in 
many instances, of thousands of 
stars, as if constituting a separate 
universe by themselves. 

789. What are NebuiwE ?* 
Clusters of stars so remote as to 

appear through common telescopes like a faint haze of 
light. 

790. How are the Nebulce distinguished ? 

Into Resolvable, Irresolvable, Planetary, Stellar, and 
Annular. 



* Neb'-u-la, singular ; Neb-u-lce, plural. 

_ il 

14* 




162 



PRIMARY ASTRONOMY. 



PLANETARY NEBU 



791. What are Resolvable Nebula ? 

Clusters, the light of whose individual stars appears 
blended through ordinary instruments, but which can 
be resolved into distinct stars by telescopes of higher 
power. 

792. What are Irresolvable Nebula 1 

Faint patches of light, formerly supposed to be vast 
fields of unorganized matter, in a high state of rarefaction. 

793. What has Lord Ross announced in regard to these bodies 7 
That nearly 200 nebulae, hitherto considered irre- 
solvable, were easily separated into stars by his mag- 
nificent telescope. 

794. What does this seem to prove as to this class of Nebula ? 
That they could all be resolved into distinct stars, if 

we had telescopes of sufficient power. 

795. What are Planetary Neb- 
ula ? 

Clusters so nearly round as 
to resemble planets through or- 
dinary telescopes. 

796. What are Stellar Nebula ? 
Such as seem to have a bright 

star at or near their center. 

797. Where are these Stars probably situated ? 
In the direction of the nebulae, 

but between them and the ob- 
server. 

79§. What are Annular Nebula ? 

Clusters that have the appear- 
ance of a ring, the stars being 
much thicker around the edge 
than in the center. 




ANNULAR NEBUL*:. 




PKEtfARY ASTKONOMY. 



163 



799. What is the Galaxy or Milky Way of our own Firmament ? 
It is a zone of light surrounding the heavens, which is 

found by the telescope to consist of countless myriads of 
stare. 

800. How do astronomers account for the vast number of small 
Stars in this Belt ? 

They suppose our cluster to be in the form of a lens 
or oblate spheroid very much flattened ; and that the 
Milky Way is an edgewise view from a position near the 
center. 

r , „, ... SHAPE OF OFR OLFSTER. 

[1. The annexed cut is a repre- 
sentation of the great stellar cluster, 
immediately surrounding the Solar 
System. It may be regarded as a 
side view of the cluster. Let the 
star near S represent the Sun. and 
imagine the most distant planets 
and comets to revolve between S 
and the star. Then if a person 
upon the Earth near the star were 
to look out of the cluster toward 
the eye of the reader, or the back 
of the book, the stars would appear 
large and scattering ; but if he 
looked off in the direction of the 
edge of the cut, they would appear 
much more numerous, constituting 
a belt of small stars around the 
heavens. 

2. On the left is seen an opening 
intended to represent a division 
that is seen extending for some 
distance in the zone of the Milky 
Way. 

3. It is supposed that if we 
could place ourselves at a distance 
beyond the most remote star in 
this immense cluster, and take an 
edgewise view of it as a whole, it 
would appear much as here rep- 
resented—the division in the line of the Milky Way being again shown on the left.] 

§01. Where are the Nebula supposed to be situated 1 
Entirely beyond the great cluster composing our own 
immediate firmament. 

§Q£. How do they appear through the most powerful Telescopes ? 




164 



PEIMAEY ASTKONOMY. 



They are found to be vast collections of glowing stars. 

§03. What are they supposed to be ? 

Clusters like that in the midst of which the Solar 
System is found, but so remote as to appear like faint 
patches of light. 

804. What, then, is the supposed structure of the Universe ? 

It is supposed to consist of vast distinct clusters, at 
immense distances from each other, and composed of 
stars, each of which is a Sun, surrounded by his own 
retinue of revolving worlds. 

[Let A represent our own cluster, supposed structure of the universe. 
with the Sun and Solar System some- 
where in its bosom. Then the nearest 
groups would appear as clusters, the 
next nearest like resolvable nebulas, and 
the more remote like irresolvable neb- 
ulae. But to an eye that could take in a 
wide field of immensity the several clus- 
ters would appear isolated, as represent- 
ed in the cut. At least these are the 
conclusions to which astronomers arrive 
by observations upon the nebulas in the 
far-distant heavens.] 

805. How would things ap- 
pear if we were to pass out of 
our own cluster, and to go to one 
of those Nebula ? 

As we passed star after star, on our way to the 
borders of our cluster, they would swell to the magni- 
tude of Suns, and again diminish to stars ; while our 
own Sun would gradually dwindle to a star, and finally 
disappear. As we left our cluster, it also would con- 
tract, while the distant nebulae expanded as we ap- 
proached and entered them, till at length we should find 
ourselves surrounded by a new firmament of constella- 
tions, and our own cluster would appear only as a dis- 
tant nebula. 




PRIMARY ASTRONOMY. 165 

LESSON XXXYIII. 

OF THE ATMOSPHERE, WINDS, CLOUDS, STORMS, ETC. 

806. What is the Atmosphere ? 

It is the air we breathe, an elastic gas which sur- 
rounds the globe on every side. 

807. To what hight does it extend above the Earth? 

Its precise hight is not known, but it is supposed to 
extend from 40 to 60 miles. 

808. What keeps it so closely wrapped around the Globe ? 

The same power that keeps the waters in their place ; 
namely, gravitation. 

809. Why does not the Air get swept off from the Globe in its 
rapid motion around the Sun? 

Because there is no substance to sweep it off, the 
region through which the Earth passes being entirely 
empty. 

810. Does the Air, then, revolve with the Earth ? 

It does, both around the Sun and the Earth's axis. 

811. Is the Density of the Air the same at all elevations ? 

It is not ; but grows more rare as we ascend from the 
Earth's surface. 

812. What is Wind? 

It is air put in motion. 

813. What is the Velocity of the Air in a gentle, pleasant Wind? 
From four to £.vq miles an hour. 

814. What of Brisk or High Winds? 
From fifteen to fifty miles an horn*. 

815. What is the Velocity of the Air in a Storm? 
From fifty to sixty miles an hour. 

816. What is the Velocity in a Hurricane ? 
From eighty to one hundred miles an hour. 



166 PEIMAEY ASTKONOMT. 

§17. What is the cause of Winds, Storms, and Hurricanes? 
The influence of heat, causing bodies of air to rise, 
and other air to rush in to supply its place. 

[Whenever air is heated it expands, and becomes lighter than cold air, so that the 
tendency is to ascend. It is this which causes flame and smoke to ascend. On this 
account, also, if a large fire take place when the air appears perfectly still, the wind 
will seem to blow in every direction toward it.] 

§18. What are Clouds 1 

A collection of misty vapors suspended in the air. 

§19. How high are the Clouds 1 

They range from two miles to half a mile, according 
to their density and weight. 

§20. Of what benefit are Clouds to us 1 

They often screen us from the oppressive heat of the 
Sun, and convey water from the rivers and oceans, and 
pour it down in showers upon the Earth. 

§21. What is Rain? 

"Water condensed, or collected into drops by attrac- 
tion, and falling from the clouds. 

§22. What is Hail? 

Drops of rain frozen on their way from the clouds to 
the Earth. 

§23. What is Snow ? 

Particles of clouds frozen before being condensed into 
drops, and falling to the Earth. 

§24. What is Lightning ? 

The passage of a fluid called electricity, from one 
cloud to another, or from the clouds to the Earth. 

§25. What is Thunder % 

A sudden shock given to the atmosphere by the pas- 
sage of electricity through it. 

§26. Why do we generally see the Lightning before we hear the 
Thunder ? 



PK1MARY ASTRONOMY. 167 

Because the velocity of light is much greater than 
that of sound. 

827. Is there any danger after the flash of Lightning is past, 
though we have not heard the Thunder ? 

There is not. It is the lightning that does the harm, 
and not the thunder. 

828. What is the Aurora Borealis, or Northern Lights? 

A reddish, unsteady light, that is sometimes seen in 
the North. 

829. Is the cause of this Light known ? 

It is not ; but it is generally supposed to be produced 
oy electricity, occultation of a star. 

830. What is meant by the Occulta- 
tion of a Star? 

It is when the Moon passes be- 
tween the Earth and a star, and 
for a time hides it from view. 

[The cut represents the New Moon just about to 
occult the star on the left.] 

831. What are " Shooting Stars?" 
Meteors that shoot from the sky downward toward 

the Earth, like stars falling from their spheres. 

832. How are they generally seen ? 

One at a time, and only in the night. 

833. Do they ever fall in great numbers? 

They do. From two o'clock in the morning till day- 
light, on the 13th of November, 1833, the whole heav- 
ens were filled with fiery particles, and streaks of light 
darting downward from the sky. 

834. Is it knoivn what these Meteors are ? 
It is not. 

835. Where are they supposed to come from ? 
From the regions beyond our atmosphere. 




168 



PRIMARY ASTRONOMY. 



A METEORIC SHOWER. 




836. How are they supposed to be set on fire ? 
By friction, in passing with great velocity through the 
atmosphere. 



A LARGE METEOIl. 



837. Do they always ap- 
pear small, as represented in 
the above cut ? 

They do not. Me- 
teors of great size have 
been known to traverse 
the atmosphere, and to 
explode with a loud re- jj 
port. 

838. Is any substance ever found belonging to Meteors 1 
What are called Meteoric Stones, and masses of iron, 

have fallen from the sky at various periods, and on 
almost every part of the globe. 














^ ... - 



rP 7 « 



v. i a « 




"^r A 5 



& 



,V" sV /* V 



'o 









aV <p 



^ y % 









^ ^ ", 



p 






v'- 



--> / 









* o>- 



<-> 







■+*. <P 



& 












■V 



^ 















Oo 


















jj JjTftflWWj^J 



